Light

Point a torch at a mirror in a dark room and something perfectly predictable happens every time: the beam bounces off at a precise angle, never at random. Point the same torch at a sheet of white paper instead, and the beam scatters every which way, which is exactly why you can read this page from any seat in a room lit by a single bulb. Both events are reflection, governed by the very same two laws — yet one produces a sharp mirror image and the other lets you see printed words from any angle at all. This chapter is built around one question: what happens when light meets a surface, and how does that single event let us see anything at all, from the page in front of you to the moon at night? The chapter answers that question in two connected halves. The first half is the physics of reflecting surfaces: the two laws of reflection, why smooth surfaces form images while rough ones merely scatter light usefully, why light bouncing off one mirror can bounce again off a second one — exactly how a periscope and a kaleidoscope work — and why white sunlight breaks apart into a rainbow of colours when it reflects through a glass-and-water arrangement. The second half turns the question inward: the eye itself is the instrument that receives all this reflected and emitted light, and the chapter walks through its parts — cornea, iris, pupil, lens, retina, optic nerve — to explain how an image lands inside your own head, why there is a small blind spot you never notice, why movies appear to move at all, and how people with no useful eyesight read and write fluently using the Braille system. By the end you should be able to state and apply the two laws of reflection to any ray diagram, explain multiple images and dispersion, label a diagram of the human eye, and reason clearly about why some things are luminous and others are merely illuminated.

Key takeaways

We see an object only when light from it enters our eyes — either light it gives off itself (luminous objects like the Sun or a flame) or light it reflects from elsewhere (illuminated objects, almost everything else, including the Moon).
First law of reflection: angle of incidence equals angle of reflection, $\angle i = \angle r$ , both measured from the normal, the line perpendicular to the surface at the point of incidence.
Second law of reflection: the incident ray, the reflected ray and the normal all lie in one single flat plane.
A smooth surface gives regular reflection (parallel rays stay parallel, forming a clear image); a rough surface gives diffused reflection (rays scatter) — the laws hold at every point in both cases, so diffused reflection is not a failure of the laws.
A plane mirror forms a virtual image: erect, same size, as far behind the mirror as the object is in front, cannot be caught on a screen, and laterally inverted (left and right swapped).
Reflected light can be reflected again by a second mirror — the basis of a periscope (two parallel 45° mirrors) and the endless patterns of a kaleidoscope (angled mirrors, producing multiple images).
Two mirrors inclined at angle $\theta$ produce $n = \dfrac{360\degree}{\theta} - 1$ images (when $360\degree/\theta$ is a whole number); parallel mirrors ( $\theta = 0\degree$ ) form, in principle, infinite images.
White sunlight splits into seven colours through a glass-and-water prism arrangement, a phenomenon called dispersion — also how a natural rainbow forms.
The eye focuses light through the cornea, pupil (size set by the iris) and lens onto the retina, where cones (bright light, colour) and rods (dim light) send signals via the optic nerve; the nerve's exit point has no light-sensing cells, the blind spot.
Key idea: light lets us see only by reaching our eyes directly or after reflecting off a surface, and the same two laws of reflection — equal angles, and everything in one plane — explain a plane mirror's image, a periscope, a kaleidoscope, and how the eye's own parts capture that light to form vision.

The core insight

Every visible object sends light to the eye directly (luminous) or by reflection (illuminated), and reflection always obeys two laws — angle of incidence equals angle of reflection, with incident ray, reflected ray and normal in one plane — laws that alone explain plane-mirror images, multiple images from inclined mirrors, periscopes, kaleidoscopes and dispersion, while the eye's cornea, iris, pupil, lens and retina turn that same reflected and emitted light into the sensation of sight.

What this chapter covers

The chapter opens by asking what actually makes an object visible, distinguishing luminous objects (which emit their own light) from illuminated objects (which we see only because they reflect light from elsewhere). It then develops the two laws of reflection from a hands-on torch-comb-mirror activity, and uses them to explain regular reflection (smooth surfaces, clear images) versus diffused reflection (rough surfaces, scattered light) — stressing that both obey the same laws. From there it shows that reflected light can strike a second mirror and reflect again, explaining the periscope, and that two mirrors set at an angle create multiple images, the working principle of the kaleidoscope. A short section on dispersion shows that sunlight is a mixture of seven colours. The second half turns to the human eye: its structure, the blind spot, persistence of vision, care of the eyes and vitamin A, and finally the Braille system.

The core ideas

What makes an object visible

Eyes alone cannot see anything in complete darkness — vision needs light to actually enter the eye from the object being looked at. Objects that give off their own light, such as the Sun, a burning candle flame, or an electric bulb, are called luminous objects. Almost everything else we see is not luminous at all — it simply reflects light that originally came from a luminous source. Objects seen this way, such as this page, a chair, or the Moon (which shines only because it reflects sunlight), are called illuminated objects. This distinction answers a common opening puzzle directly: you cannot see an object in a completely dark room, whether it sits inside or just outside the room, because no light is present to be emitted or reflected into your eye in either case.

The two laws of reflection

Hold a comb with a single narrow opening in front of a torch so only a thin ray of light passes through, and let that ray strike a plane mirror placed on a sheet of paper. The ray arriving at the mirror is the incident ray; the ray leaving after striking the surface is the reflected ray. Draw a line perpendicular to the mirror at the exact point the ray strikes it — the normal. Careful measurement of the angle between the incident ray and the normal (the angle of incidence, $\angle i$ ) and between the reflected ray and the normal (the angle of reflection, $\angle r$ ) shows they are always equal, however the torch is tilted.

\angle i = \angle r

where: $\angle i$ is the angle of incidence and $\angle r$ is the angle of reflection, both measured from the normal, never from the mirror's surface. This is the first law of reflection. The second law, found by repeating the activity on a sheet that can be bent at the point of incidence, is that the incident ray, the normal, and the reflected ray all lie in one flat plane — bend the paper so the reflected ray's portion leaves that plane, and the reflected ray simply disappears from view.

Figure

A ray diagram on a horizontal mirror line showing an incident ray arriving from the upper left, a dashed normal drawn perpendicular to the mirror at the point of incidence, and a reflected ray leaving toward the upper right, with the angle of incidence and the angle of reflection marked equal on either side of the normal

Regular reflection and diffused reflection

The laws of reflection hold at every point of any surface, smooth or rough — but what happens to a whole bundle of parallel rays depends on the surface's texture. A smooth surface, such as a plane mirror or still water, keeps a bundle of parallel incident rays parallel after reflection, because every point has its normal pointing the same way. This is regular reflection, and it forms a clear image. A rough surface, such as cardboard or a plastered wall, has microscopic bumps, so the normal points in a slightly different direction at every point; parallel rays striking it leave in many directions. This is diffused or irregular reflection. It is not a breakdown of the laws — each ray still obeys $\angle i = \angle r$ at its own point; it is only the irregular shape of the surface that scatters the bundle. Diffused reflection is, in fact, what lets you see most things at all — light bouncing off a page or a wall in every direction is why it can be seen from any position in a room.

Images in a plane mirror

A plane mirror always forms exactly one image, with four features worth listing together: it is erect, the same size as the object, it appears exactly as far behind the mirror as the object stands in front, and it cannot be captured on a screen — hold a sheet of paper where the image appears to be and nothing lands on it, because the rays only appear to come from that point without actually meeting there. An image formed only by this apparent meeting of rays is called a virtual image. A mirror image also shows lateral inversion: raise your right hand in front of a mirror and the image appears to raise its left hand — left and right are interchanged, though up and down are not.

Reflected light reflected again: periscopes and kaleidoscopes

A ray that reflects off one mirror does not stop obeying the laws just because it has bounced once — if it strikes a second mirror, it reflects again just as predictably. A periscope uses two plane mirrors parallel to each other, each tilted at 45° to the tube that holds them, so light entering the top mirror reflects down and then a second time straight into the eye — how a submarine crew sees above the surface. A kaleidoscope joins three mirror strips into a triangular tube with loose coloured glass at one end; every tilt rearranges the pieces, and the multiple reflections turn that small arrangement into large, symmetric patterns — no two views are ever the same.

Dispersion of sunlight

Sunlight looks white, but it is not a single colour — it is a mixture of seven colours blended together. Fill a shallow bowl (katori) with water, place a small plane mirror tilted inside it, and set it in direct sunlight near a window so the reflected light falls on a white wall. The patch of light is not white; it shows a clear band of colours. The mirror and water together act like a prism, bending each colour in white light by a slightly different amount, spreading them into a visible band. This splitting of light into its component colours is called dispersion, and a natural rainbow is dispersion on a vastly larger scale, with raindrops acting as the prism.

The structure of the human eye

Light reaching the eye must be focused and sensed before the brain registers an image, and each part plays a specific role. Light first passes through the cornea, the transparent, curved front of the eye's tough outer coat. Behind it is the iris, a dark, muscular, coloured ring that gives eyes their colour, with a small central opening, the pupil, whose size the iris controls: it widens in dim light and narrows in bright light. Just behind the pupil sits the lens, thicker at its centre, which focuses light onto the retina, a light-sensitive layer lining the back of the eyeball. The retina holds two kinds of nerve cells: cones (bright light, colour) and rods (dim light). Signals travel to the brain along the optic nerve.

\text{Cornea} \rightarrow \text{Pupil (size set by iris)} \rightarrow \text{Lens} \rightarrow \text{Retina (rods and cones)} \rightarrow \text{Optic nerve} \rightarrow \text{Brain}

where: each arrow marks a stage light passes through, in order, on its way from outside the eye to the sensation of sight registered in the brain; no stage can be skipped, and a problem at any one stage (a cloudy lens, a damaged retina) can impair vision even if every other stage works perfectly.

The blind spot and persistence of vision

At the exact point on the retina where the optic nerve leaves the eye there are no light-sensitive cells, so no vision is possible at that one small spot — the blind spot. You never notice it in daily life mainly because your two eyes cover for each other and your brain fills the small gap with surrounding detail, but it can be demonstrated with a simple mark-and-cross activity: close one eye, look steadily at a cross while a nearby round mark is slid toward or away from you, and at one particular distance the round mark vanishes. A second, unrelated property is that the impression of an image does not vanish from the retina the instant light stops — it lingers about $1/16$ of a second, called persistence of vision. Still images flashed faster than 16 per second are perceived as smooth motion, which is why films, shown at 24 pictures per second, read as fluid movement rather than a flicker of separate pictures.

Care of the eyes, and visually impaired persons

The eye is delicate: use spectacles if advised; avoid too little light (eyestrain, headaches) and too much light (can injure the retina); never look directly at the Sun; never rub the eyes, and wash out dust with clean water instead; and read at the normal, comfortable distance of about 25 cm. A deficiency of vitamin A is a well-known cause of eye trouble, most commonly night blindness; carrots, green leafy vegetables, milk, eggs and papaya are good sources of it. Some people have very limited vision or none at all; they typically develop touch and hearing more sharply, and can be supported with non-optical aids (Braille, audio recordings, talking calculators) and optical aids (corrective lenses, magnifiers). The most widely used resource is Braille, developed by Louis Braille and published in 1821: 63 raised-dot patterns, each cell two vertical rows of three dots, recognised entirely by touch.

Derivation AWhy a plane mirror's image sits exactly as far behind the mirror as the object is in front, and why it must be virtual

The goal here is to show that the familiar features of a plane-mirror image — equidistant, same size, virtual, not obtainable on a screen — are not separate facts to memorise but a direct consequence of the two laws of reflection applied to two rays leaving one point on the object. This is the geometric backbone behind a person standing 1 m from a mirror seeing their image 2 m away from themselves.

Step 1. Let a point source of light $O$ stand in front of a plane mirror $PQ$ . Two rays leave $O$ and strike the mirror at two points, $A$ and $C$ . At each point, draw the normal to the mirror and apply the first law of reflection to construct the reflected rays $AB$ and $CD$ , with angle of reflection equal to angle of incidence at each point.

Step 2. Extended forward, $AB$ and $CD$ spread apart rather than meeting, since they left $O$ at different angles. Extended backward, behind the mirror, they do meet, at a point $I$ . To an eye placed where the actual reflected rays travel, light appears to come from $I$ , even though no ray ever actually passes through it. This is the definition of a virtual image — formed only by the apparent backward meeting of rays, so it can never be caught on a screen placed at $I$ , since no real light energy arrives there.

Step 3. To show $I$ is as far behind the mirror as $O$ is in front, drop the perpendicular from $O$ to the mirror at point $M$ ; the perpendicular from $I$ meets the mirror at the same point $M$ . Using the equal-angle condition at $A$ , triangles $OMA$ and $IMA$ share side $MA$ , both have a right angle at $M$ , and the equal angle of incidence and reflection at $A$ force their remaining angles equal too — so the triangles are congruent, giving $OM = IM$ : the object's and image's perpendicular distances from the mirror are equal.

Step 4. Since $O$ was any point on the object and the argument holds identically for every other point, the whole image sits behind the mirror at the same distance the object sits in front, is the same size (each point maps to a corresponding point at equal distance), and is erect, since up stays up throughout the construction. A separate, purely observational check — comparing your own right ear's position to its mirrored position — confirms the remaining property, lateral inversion, which follows from the same left-right symmetry of the reflection.

Go deeper · Derivation B · 1 minThe number of images formed by two mirrors inclined at an angle, from the kaleidoscope to the infinite parallel-mirror case

Derivation BThe number of images formed by two mirrors inclined at an angle, from the kaleidoscope to the infinite parallel-mirror case

The goal here is to turn the loose observation that tilting two mirrors changes how many images you see into one usable formula covering every case in the chapter — a right angle, 60°, and even two mirrors set exactly parallel, the arrangement behind the endless receding images seen in a lift or a barber's shop.

Step 1. Start from the single-mirror case as the anchor. One plane mirror alone forms exactly one image, since there is only one reflecting surface for light to bounce off before reaching the eye.

Step 2. Introduce a second mirror at an angle $\theta$ to the first. Light can now reach the eye by several distinct paths — off mirror 1 alone, off mirror 2 alone, off mirror 1 then mirror 2, off mirror 2 then mirror 1, and further alternating reflections — each path producing one more image. As $\theta$ shrinks, light has room to bounce back and forth more times before escaping the wedge, so more images form; as $\theta$ grows toward 180°, fewer bounces are possible and the count shrinks back toward one.

Step 3. This bounce-counting logic gives the standard relation for two inclined mirrors, valid whenever $360\degree/\theta$ is a whole number:

n = \frac{360\degree}{\theta} - 1

where: $n$ is the number of images and $\theta$ is the angle between the mirrors, in degrees. Two mirrors at $\theta = 90\degree$ (the coin activity) give $n = 360/90 - 1 = 3$ images, matching the observed three images of a coin between perpendicular mirrors.

Step 4. Push the formula to its limit. As $\theta \to 0\degree$ (mirrors nearly parallel), $360\degree/\theta$ grows without bound, so $n \to \infty$ — matching the observation that a candle between two exactly parallel mirrors produces, in principle, an endless series of images receding one behind the other, each fainter than the last since no mirror is perfectly reflective. The same formula that counts three images at $90\degree$ therefore also explains why parallel mirrors appear to produce images without end.

Worked examples

Board level - Exercise 13: angle of incidence when the reflected ray is at 90° to the incident ray

What is the angle of incidence of a ray if the reflected ray is at an angle of 90° to the incident ray?

Step 1. Draw the normal, and let the angle of incidence be $\angle i$ . By the first law, angle of reflection $\angle r = \angle i$ .

Step 2. The normal splits the total angle between the incident and reflected rays into $\angle i$ and $\angle r$ , so that total angle is $\angle i + \angle r$ .

Step 3. Substitute the given 90° condition and $\angle r = \angle i$ : $\angle i + \angle i = 90\degree$ , so $2\angle i = 90\degree$ .

Step 4. Solve: $\angle i = 45\degree$ . Answer: the angle of incidence is 45° (and the angle of reflection is also 45°).

Board level - counting images formed by two mirrors at 60°, extending Activity 13.5

Two plane mirrors are set up with their edges touching, inclined at 60°, and a small object is placed between them. How many images of the object are formed?

Step 1. Identify the formula for two inclined mirrors: $n = \dfrac{360\degree}{\theta} - 1$ , valid here since $360\degree/60\degree$ is a whole number.

Step 2. Substitute $\theta = 60\degree$ : $n = \dfrac{360\degree}{60\degree} - 1 = 6 - 1$ .

Step 3. Complete the arithmetic. Answer: 5 images are formed — more than the 3 images formed at a right angle (90°), confirming that images multiply as the angle shrinks.

Higher-order thinking (HOTS / olympiad-style) - Exercise 15: a ray bounced between two perpendicular mirrors

Two mirrors meet at right angles. A ray of light is incident on one of them at an angle of incidence of 30°. Find the direction of the ray after it reflects from the second mirror, and explain what you notice about its direction compared with the original incident ray.

Step 1. Let the ray strike mirror 1 at point $A$ with angle of incidence $i_1 = 30\degree$ , reflecting at $30\degree$ too, and travel on to strike mirror 2 at point $B$ with unknown angle of incidence $i_2$ .

Step 2. Consider the triangle formed by $O$ , where the mirrors meet, and points $A$ and $B$ . Since the mirrors are at right angles, the angle at $O$ is $90\degree$ . The angle at $A$ , between the mirror and ray $AB$ , is $90\degree - i_1$ ; likewise the angle at $B$ is $90\degree - i_2$ .

Step 3. The three angles of triangle $OAB$ add to $180\degree$ : $90\degree + (90\degree - i_1) + (90\degree - i_2) = 180\degree$ , so $i_1 + i_2 = 90\degree$ .

Step 4. Substitute $i_1 = 30\degree$ : $i_2 = 60\degree$ . Answer: the ray strikes mirror 2 at an angle of incidence of 60° and reflects off it at 60° too. Tracing the geometry further shows this final ray travels exactly parallel to the original incident ray but in the opposite direction — a total turning of 180° — a result true for any starting angle $i_1$ , which is why two mirrors at a right angle work as simple retroreflectors, such as the safety reflectors on bicycles that send a car's headlight beam straight back toward the driver regardless of the angle it arrived at. Timing note: about 3 to 4 minutes, most spent on the triangle-angle argument in Step 2 and Step 3. The trap to avoid is assuming the two angles of incidence must be equal — they generally are not; what stays fixed is their sum, $i_1 + i_2 = 90\degree$ , and this is what forces the outgoing ray to reverse direction by exactly 180°.

Common traps

Thinking diffused reflection breaks the laws of reflection. It does not. Every ray on a rough surface still obeys $\angle i = \angle r$ at its own point; only the irregular tilt of the surface scatters the bundle.

Measuring angles from the mirror's surface instead of from the normal. Both angles are always defined from the normal — a common slip is reading the angle between the ray and the mirror itself, the complement of the correct angle.

Believing a plane mirror's image is real because it looks so clear. It is virtual: the reflected rays only appear to diverge from a point behind the mirror; no screen placed there will ever catch the image.

Confusing lateral inversion with a size or shape change. A plane mirror image is exactly the same size and shape — lateral inversion only swaps left and right; it does not enlarge, shrink, or flip the image.

Applying $n = 360\degree/\theta - 1$ without checking it gives a whole number. The clean formula applies directly only when $360\degree/\theta$ is a whole number, as in every example here.

Assuming luminous means bright and illuminated means dull. The distinction is about the source of light, not brightness: a faint candle flame is luminous, while a brilliant sunlit wall is still only illuminated.

Common trapDiffused reflection is not a failure of the laws of reflection

The single most common misconception in this chapter is thinking that when light scatters off a rough surface, the laws of reflection have somehow stopped applying. They have not. The two laws — equal angles of incidence and reflection, and all three (incident ray, normal, reflected ray) in one plane — hold true at every individual point of contact on any surface, smooth or rough. What differs between a mirror and cardboard is only the surface's shape at a microscopic level: a mirror is flat enough that the normal points the same way everywhere, so a bundle of parallel rays stays parallel after reflecting (regular reflection). A rough surface has its normal tilted differently point to point, so each ray, while still perfectly obeying $\angle i = \angle r$ locally, travels off in a different overall direction than its neighbours, scattering the bundle (diffused reflection). Whenever a question asks you to explain diffused reflection, the correct answer always credits the irregularity of the surface, never a suspension of the laws themselves.

You should be able to…

Distinguish luminous from illuminated objects, and explain why you cannot see anything in complete darkness.
State both laws of reflection precisely, using the normal (not the mirror's surface) to measure angles.
Describe an activity showing the incident ray, reflected ray and normal all lie in one plane.
Explain regular and diffused reflection, and firmly state why diffused reflection does not mean the laws have failed.
List all four features of a plane mirror's image, and explain, using a ray diagram, why it is virtual and equidistant.
Explain lateral inversion with an everyday example, such as raising a hand in front of a mirror.
Explain how a periscope and a kaleidoscope work, and calculate images from two inclined mirrors using $n = 360\degree/\theta - 1$ .
Explain dispersion and describe the mirror-in-water activity that demonstrates it.
Label a diagram of the human eye and explain each part's role, including the blind spot and persistence of vision.
List eye-care practices, the vitamin A and night-blindness link, and the basic structure of Braille.

Ask the tutor: if you want to work through more two-mirror ray-tracing problems like the right-angle retroreflector, or trace exactly how a periscope's two mirrors redirect a beam of light step by step, ask your tutor to draw it out with you.

Why can I see a wall from any angle in a room but only see a clear image of myself in a mirror when I stand right in front of it

Both reflect light, but very differently. The wall's surface is rough at a microscopic level, so parallel light striking it scatters in every direction — diffused reflection — meaning some of that light reaches your eye no matter where you stand. A mirror's surface is smooth, so it keeps a bundle of parallel rays parallel after reflecting — regular reflection — meaning the light forming your image travels off in one specific, predictable direction. To see your image you have to stand where that reflected light actually goes, roughly in front of the mirror; step too far aside and it simply misses your eyes.

If my image in the mirror is virtual and cannot be captured on a screen, why does it look so solid and real to me

A virtual image looks completely convincing because your eye and brain do not care whether light rays actually met at a point or only appear to have come from it — either way, the rays entering your eye diverge exactly as if they originated there, and your visual system reconstructs an image accordingly. The only physical difference is that a screen placed at the apparent location receives nothing, because no light energy is actually converging there; only your own eye's lens can turn that diverging light back into a sharp image on your retina.

Why do my eyes not notice the blind spot all the time, if it is a real gap in my vision

Three things quietly cover for it. Each eye's blind spot sits at a slightly different position in your combined field of view, so with both eyes open the other eye almost always has good information exactly where one eye's blind spot falls. Even with one eye closed, your brain fills the small gap using the colour, texture and detail from the surrounding area, so you perceive a smooth scene even though a small patch is genuinely never sensed. Your eyes are also rarely perfectly still, so the blind spot is not fixed on one part of a scene for long. The mark-and-cross activity works precisely because it forces one eye to stay fixed on a target while the other is closed, removing all three safeguards.

Is the number of images formula for two mirrors, n equals 360 over theta minus 1, always exactly correct

It gives the exact number of images whenever 360° divided by the angle between the mirrors comes out to a whole number, covering every case in this chapter, such as 90°, giving 3 images, or 60°, giving 5. When the angle does not divide evenly into 360°, the object's exact position starts to matter and the count can differ slightly from the plain formula; this refinement goes beyond Class 8 needs, so for chapter-level questions apply the formula directly whenever the given angle divides 360° evenly, exactly how the chapter's own examples are set up.