Light: Shadows and Reflections

Why This Matters

Think about the last time you sat in the Sun. Did you see a dark shape on the ground that copied your body? That was your shadow. It follows you everywhere on a sunny day.

Now think about the last time you looked into a mirror to comb your hair. You saw your own face looking back at you. How did that happen?

Shadows and mirror reflections feel like everyday magic. We see them so often that we stop asking why. But there is one simple idea behind both of them: light travels in straight lines.

In this chapter we will go slowly. We will find out which things give their own light. We will learn why some things let light through and some do not. We will see exactly why a shadow forms, and why you can see yourself in a mirror. By the end, these everyday wonders will make complete sense.

The Big Idea

Light travels in straight lines. That one fact explains almost this whole chapter. When something blocks light, the light cannot bend around it — so a dark patch called a shadow forms behind it. When light hits a shiny, flat surface like a mirror, it bounces off in a new straight direction — this bouncing is called reflection, and it lets you see your own face. Some objects make their own light (the Sun, a flame), while most objects only shine because light from somewhere else bounces off them. Keep “light goes straight” in your mind, and the rest will follow.

Let’s Break It Down

Light travels in straight lines

Here is the most important idea in the whole chapter. Light always travels in a straight line. It does not curve or bend on its own as it moves through air.

A single straight path of light is called a ray. Think of a ray as a thin, straight line that light follows, like a tightly pulled string.

How do we know light goes straight? Try this. Take a straight pipe and look at a candle flame through it. You can see the flame. Now bend the pipe. Can you still see the flame? No. The light from the flame cannot turn the corner of the bent pipe to reach your eye. It can only travel straight.

Figure 11.1 below shows this. A light source sends out straight rays. Through a straight pipe you see the flame; through a bent pipe you do not.

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This straight-line behaviour has a special name. Rectilinear propagation of light just means “light spreads out in straight lines”. “Recti” means straight and “linear” means line. Do not worry about the long word — just remember the simple idea: light goes straight.

Luminous and non-luminous objects

Where does light come from in the first place? Some things make their own light. Some do not.

An object that makes its own light is called a luminous object. The word “luminous” comes from a word meaning “light”. The Sun is luminous — it gives out its own light. A burning candle, a glowing tube light, a flash of lightning, and even a firefly are all luminous.

An object that does not make its own light is called a non-luminous object. We can only see these things because light from somewhere else falls on them and bounces off into our eyes. A book, a chair, a wall, your friend’s face — all non-luminous.

Here is a surprising one. The Moon is non-luminous. It does not make its own light at all. The bright moonlight you see at night is really sunlight. Sunlight falls on the Moon, bounces off it, and travels to your eyes. So moonlight is just borrowed sunlight.

Transparent, translucent, and opaque materials

Now let’s see what happens when light meets an object in its path. It depends on the material the object is made of. There are three cases.

A transparent material lets almost all the light pass through it. You can see clearly through it. Clear glass and clean water are transparent. Look through a glass window — you see the street outside sharply.

A translucent material lets some light pass through, but not all. You can see light coming through, but you cannot see things clearly on the other side. Tracing paper, frosted glass, and thin cloth are translucent. Light gets through, but the view is blurry.

An opaque material lets no light pass through at all. You cannot see through it. A wooden door, a brick wall, a metal sheet, and cardboard are opaque.

Figure 11.2 below shows all three side by side. Notice how much light reaches the screen (the grey bar) in each case.

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Let’s lay these three out clearly so you can compare them at a glance.

Type	How much light passes?	Can you see through it?	Examples
Transparent	Almost all light passes	Yes — clearly	Clear glass, clean water, air
Translucent	Some light passes	Only blurry shapes	Tracing paper, frosted glass, thin cloth
Opaque	No light passes	No — not at all	Wood, brick wall, metal, cardboard

How shadows form — and why a shadow copies the object’s outline

Now we can answer a question you may never have thought to ask: why does a shadow form at all?

Remember our big idea: light travels in straight lines. So when an opaque object comes in the path of light, it blocks the light. The light cannot bend around the object. Behind the object there is a region where no light reaches. That dark, light-free region is the shadow.

You need three things to make a shadow you can see:

• a source of light (like the Sun or a torch),
• an opaque object to block the light, and
• a screen to catch the shadow (a wall, the floor, or the ground).

But here is the deeper “why”: why does the shadow have the same shape as the object? Why is the shadow of a ball round, and the shadow of your hand shaped like your hand?

Think about the rays of light that just graze the edges of the object. These edge rays decide the outline of the dark region. Because every ray travels straight, the edge of the shadow lines up exactly with the edge of the object. So the shadow ends up with the object’s outline. Figure 11.3 below makes this clear.

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Features of a shadow. Now that you understand how it forms, here are the things that are always true about shadows:

• A shadow is always on the side away from the light. The object blocks the light, so the dark region is behind it, opposite the source.
• A shadow shows only the outline (shape) of the object. It does not show colour or any detail. A shadow is always dark, no matter what colour the object is.
• The shadow’s size can change. Move the object closer to the light, and its shadow grows bigger. Move it closer to the screen, and the shadow gets smaller and sharper.
• Opaque objects make dark, clear shadows. Translucent objects make lighter, fainter shadows (because some light still gets through). Most transparent objects make almost no shadow.

Let’s put this understanding to work on a real question.

The pinhole camera

Here is a lovely use of “light travels in straight lines”. A pinhole camera is a simple device. It is a closed box with one tiny hole (a pinhole) on one side and a screen on the opposite side. Light from an object passes through the tiny hole and forms a picture (an image) on the screen.

The surprise is this: the image is upside down (inverted). If you point it at a candle, the flame appears at the bottom of the image, not the top.

Why is it upside down? Again, it is because light goes straight. A ray from the top of the flame travels in a straight line, passes through the tiny hole, and keeps going straight — so it lands on the bottom of the screen. A ray from the bottom of the flame goes straight through the hole and lands on the top. The rays cross at the pinhole. So top and bottom get swapped, and the image is upside down. Figure 11.4 below shows the crossing rays.

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A pinhole-camera image is upside down, and it also shows the real colours of the object — unlike a shadow, which is only dark.

Reflection from a mirror

You have a torch and you shine it at a flat mirror lying on the floor. A bright spot of light appears on the wall. The mirror has sent the light off in a new direction. This bouncing of light off a shiny surface is called reflection of light.

A plane mirror is just a mirror that is flat (not curved). When light hits it, the mirror does not let the light pass through and it does not soak it up. It bounces the light back in a new straight line.

Here is the key idea: reflection changes the direction of light, but the light is still travelling in straight lines. It goes straight up to the mirror, then straight off in a new direction. Figure 11.5 below shows this.

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This bouncing is also what lets you see things. When you see a wall or a friend, light has bounced off them into your eyes. A mirror just does this much more neatly, because its surface is so smooth and shiny.

Images in a plane mirror — and why they are swapped left to right

Stand in front of a flat mirror. You see your own face. That face you see is called your image. The “thing” in front of the mirror (you) is the object.

A plane mirror image has four features. Let’s go through each one.

• The image is behind the mirror. It looks as if another “you” is standing behind the glass. But nothing is really there — light only appears to come from behind the mirror.
• The image is the same size as the object. You are not bigger or smaller in the mirror. You are exactly your size.
• The image is the same distance behind the mirror as you are in front. Stand 1 metre from the mirror, and your image looks 1 metre behind it. Step back, and your image steps back the same amount.
• The image is erect (the right way up). Your head is at the top in the mirror, just like in real life. It is not upside down (unlike the pinhole-camera image).

But there is one famous catch. Raise your left hand in front of a mirror. Look carefully — the image raises the hand on the other side. Your left becomes the image’s right, and your right becomes the image’s left. This left-right swap is called lateral inversion.

Why does this happen? The mirror does not actually flip you. It simply shows each part of you straight across from where it really is. The part of you nearest your left stays nearest the same side of the mirror — but because the image is now facing you, that side reads as “right” to the facing image. So front-and-back stays the same and top-and-bottom stays the same, but left and right appear swapped. Figure 11.6 below shows a child raising one arm and the image raising the opposite arm.

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A neat real-life example: the word AMBULANCE is written backwards on the front of an ambulance. Why? So that a driver ahead, looking in their rear-view mirror, sees it laterally inverted — which makes it read the right way round, as AMBULANCE. Then they know to give way.

One more feature: a plane mirror image cannot be caught on a screen. If you hold a paper screen behind or in front of the mirror, no image appears on it. The image only seems to be behind the mirror — it is not really there.

Common Mistakes

Let’s clear up some ideas that trip up many students.

Quick Check

Let’s test what you’ve learnt with a couple of quick questions.

Practice Problems

Try each one yourself first, then tap to check the full answer.

Easy

Medium

Challenge

Summary

• Light travels in straight lines. This single idea explains shadows, the pinhole camera, and reflection. You can see a flame through a straight pipe but not a bent one.
• Luminous objects make their own light (Sun, flame, firefly). Non-luminous objects do not — they only shine by reflecting light. The Moon is non-luminous; moonlight is reflected sunlight.
• Transparent materials let almost all light through (glass, water). Translucent materials let some through (tracing paper). Opaque materials let none through (wood, brick).
• A shadow forms when an opaque object blocks straight-line light. The shadow is on the side away from the source, shows only the object’s outline (no colour or detail), and can change size.
• A pinhole camera forms an upside-down (inverted) image, because straight rays cross at the tiny hole.
• Reflection is the bouncing of light off a shiny surface like a mirror. It changes the light’s direction but the light still travels straight.
• A plane mirror image is behind the mirror, the same size as the object, the same distance behind as the object is in front, erect, and laterally inverted (left and right swapped). It cannot be caught on a screen.

What’s Next

You now know that the Moon does not make its own light — it only reflects sunlight. That naturally leads to a bigger question: how do the Sun, the Earth, and the Moon move together? Why do we get day and night, and why does the Moon seem to change shape through the month?

In the next chapter, Chapter 12 — Earth, Moon, and the Sun, you will follow exactly how these three move and how that gives us day, night, and the phases of the Moon. The idea of reflected sunlight that you learnt here will help you understand all of it.