What Is Light, Really?
Light is something we rarely think about — until we close our eyes in a dark room and suddenly can’t see anything. That tells us something immediately: light is necessary for vision. But there’s more to it than that.
Light is a form of energy that travels in straight lines. It is produced by sources (called luminous objects) and bounces off things that don’t produce their own light (called non-luminous objects). When light bounces off a non-luminous object and enters our eyes, we see that object.
We also know light travels incredibly fast — about 3 × 10⁸ m/s in vacuum (air is close enough for Class 7-8 purposes). That’s so fast that light from the Sun reaches Earth in roughly 8 minutes, covering 150 million kilometres.
For Class 7 and Class 8 CBSE, you need to understand three big ideas about light: how it travels, how it reflects, and how it bends (refracts) when it moves from one material to another. Once you understand the logic behind each, the questions almost solve themselves.
Key Terms and Definitions
Luminous objects — objects that produce their own light. The Sun, a burning candle, a glowing electric bulb, a firefly.
Non-luminous objects — objects we see only because light falls on them and reflects. The Moon, a table, a book. The Moon is the classic NCERT example — it shines because it reflects sunlight.
Opaque objects — do not allow light to pass through. A wooden door, a brick wall.
Translucent objects — allow some light to pass through, but you can’t see clearly through them. Frosted glass, oiled paper, butter paper.
Transparent objects — allow light to pass through completely and you can see clearly. Clear glass, clean water.
Ray of light — the path along which light travels, shown as an arrow in diagrams.
Beam of light — a collection of rays. A torch produces a beam.
Shadow — a dark region formed when an opaque object blocks light. The shape of a shadow depends on the shape of the object and the position of the light source.
Reflection — bouncing back of light from a surface.
Refraction — bending of light when it passes from one medium to another.
Rectilinear Propagation of Light
Rectilinear propagation means light travels in a straight line. This is the foundation of everything else.
We can verify this with a simple experiment: take three cardboard pieces, make a small hole in each, and line them up. Light passes through only when all three holes are perfectly aligned. The moment you shift one card, the light is blocked. This proves light cannot bend on its own — it goes straight.
Shadows and eclipses both exist because light travels in straight lines. If light could bend around objects, we’d never have a proper shadow.
Pinhole Camera
A pinhole camera is a direct application of rectilinear propagation. It’s a box with a tiny hole on one side and a screen (or tracing paper) on the other.
When you point it at a bright object:
- Light from the top of the object passes through the hole and hits the bottom of the screen
- Light from the bottom of the object passes through the hole and hits the top of the screen
The result is an inverted (upside-down) and laterally reversed image on the screen. This happens because light travels in straight lines and crosses at the pinhole.
CBSE Class 7 loves pinhole camera diagrams. Practice drawing the ray diagram — two rays, one from the top of the object, one from the bottom, both passing through the pinhole and crossing to form an inverted image. Label clearly.
Reflection of Light
When light hits a smooth surface, it bounces back. This is reflection.
Laws of Reflection
Law 1: The angle of incidence = The angle of reflection
Law 2: The incident ray, the reflected ray, and the normal at the point of incidence all lie in the same plane.
Here, the normal is an imaginary line drawn perpendicular to the reflecting surface at the point where light hits. All angles are measured from the normal, not from the surface.
Most students measure the angle from the surface, not the normal. This is the single most common error in reflection diagrams. Always draw the normal first, then measure angles from it.
Types of Reflection
Regular (specular) reflection — occurs on smooth, polished surfaces like mirrors and still water. Parallel rays remain parallel after reflection, giving a clear image.
Diffuse (irregular) reflection — occurs on rough surfaces like walls, paper, and roads. Rays scatter in all directions. This is why you can see a wall from any angle — diffuse reflection scatters light in every direction.
Diffuse reflection is why a rough road isn’t blinding during the day. If roads had regular reflection like mirrors, every car’s headlight would blind you permanently.
Images in a Plane Mirror
A plane mirror (flat mirror) produces a specific kind of image:
- Virtual — the image appears to be behind the mirror, where light rays never actually reach
- Erect — same orientation as the object (not upside-down)
- Laterally inverted — left and right are swapped (why text appears reversed in mirrors)
- Same size as the object
- Same distance behind the mirror as the object is in front
CBSE Class 8 marking scheme: if asked to list properties of a mirror image, give all five points above. Each is worth half a mark in a 2-mark question.
Why Can We See Around Corners Using Multiple Mirrors?
A kaleidoscope uses multiple mirrors at angles to create beautiful patterns. A periscope uses two plane mirrors at 45° angles to let you see over walls or around corners. Light reflects from one mirror to another and finally into your eye.
Refraction of Light (Class 8)
When light moves from one medium to another — say, from air into water — its speed changes. And when the speed changes at an angle, the direction changes too. This bending is called refraction.
Why does it bend? Think of it like a car moving from a smooth road (fast) onto a muddy patch (slow). If one wheel enters the mud first, that side slows down and the car turns. Light does the same thing.
- Light going from less dense to more dense medium (air → water/glass): bends towards the normal
- Light going from more dense to less dense medium (water → air): bends away from the normal
A trick to remember: Dense medium → towards Normal. Both start with D… actually, easier: going INTO water (denser), light is “pulled in” towards the normal. Coming OUT of water, light “breaks free” away from the normal.
Everyday Examples of Refraction
A pencil in a glass of water appears bent at the surface. The part in water seems displaced from where it actually is.
A coin in a bowl appears to rise as you add water — the light from the coin bends as it exits water, making it appear higher than it is.
A swimming pool appears shallower than it really is — light bends as it exits the water, making the bottom appear closer to the surface.
A lemon in a glass appears bigger — the water and glass act like a lens.
Dispersion of Light (Class 8)
When white light passes through a prism, it splits into its seven component colours. This is called dispersion.
The colours are (in order from least bent to most bent):
VIBGYOR → Violet, Indigo, Blue, Green, Yellow, Orange, Red
Or in reverse: ROYGBIV → Red, Orange, Yellow, Green, Blue, Indigo, Violet
Memorise VIBGYOR — Violet is bent most, Red is bent least. The rainbow appears because raindrops act like tiny prisms, dispersing sunlight.
White light is not a single colour — it is a mixture of all seven colours. The prism separates them because each colour bends by a slightly different amount.
Solved Examples
Example 1 — Easy (CBSE Class 7)
Question: Light from a torch falls on a book. The book is a non-luminous object. Explain why we can still see the book.
Solution:
The torch is a luminous object — it produces its own light.
Light from the torch hits the surface of the book. The book is non-luminous but it is also non-transparent — it reflects light in all directions (diffuse reflection, because the surface of a book is rough).
Some of this reflected light enters our eyes. Our brain processes this and we see the book.
Example 2 — Medium (CBSE Class 7-8)
Question: In a ray diagram for a plane mirror, a ray strikes the mirror surface at 30° to the mirror. What is the angle of reflection?
Solution:
The angle given (30°) is measured from the mirror surface, not from the normal.
Normal is perpendicular to the mirror. So angle of incidence = 90° − 30° = 60°.
By the law of reflection, angle of reflection = angle of incidence = 60° from the normal.
Never write “angle of reflection = 30°” here. That’s the angle from the surface, not the normal. CBSE specifically tests whether students know angles are measured from the normal.
Example 3 — Medium (CBSE Class 8)
Question: A fish is at the bottom of a pond 3 m deep. A person standing outside the pond looks at the fish. Does the fish appear at 3 m depth, more than 3 m, or less than 3 m?
Solution:
Light travels from the fish (in water, denser medium) to air (less dense medium). At the water surface, light bends away from the normal.
When this refracted light enters the observer’s eyes, the brain traces the rays back in a straight line — and the fish appears to be at the point where these backward-traced lines meet. This point is shallower than the real position.
So the fish appears at less than 3 m depth. The pond appears shallower than it is.
Example 4 — Hard (CBSE Class 8 / NTSE level)
Question: A person stands 2 m in front of a plane mirror. Their image forms 2 m behind the mirror. They now walk 0.5 m towards the mirror. What is the new distance between the person and their image?
Solution:
Initially: person is 2 m in front → image is 2 m behind. Total distance = 2 + 2 = 4 m.
After walking 0.5 m towards mirror: person is now 2 − 0.5 = 1.5 m from mirror. Image forms 1.5 m behind the mirror.
Total distance between person and image = 1.5 + 1.5 = 3 m.
Exam-Specific Tips
CBSE Class 7
- Pinhole camera diagram is high-frequency — practise it till your hand draws it automatically
- Shadow formation questions always ask you to explain using rectilinear propagation — use those exact words
- Luminous vs non-luminous distinction comes up in 1-mark MCQs
CBSE Class 8
- Reflection laws + plane mirror properties are typically 3-5 marks combined
- Refraction questions love asking about apparent depth (pool, coin, fish) — explain in terms of light bending away from normal
- Dispersion: know the order VIBGYOR and which colour bends most/least
In CBSE Science exams, when asked to “explain” something about light, always link it back to either rectilinear propagation, reflection laws, or refraction. Examiners give marks for using these specific terms.
Common Mistakes to Avoid
Mistake 1: Measuring angles from the surface, not the normal. Always draw the normal (perpendicular to surface) first. Angles of incidence and reflection are measured from the normal, not the mirror surface.
Mistake 2: Calling the Moon a luminous object. The Moon does not produce light. It reflects sunlight. The Moon is non-luminous. This is a very common 1-mark error.
Mistake 3: Saying refraction only occurs in water. Refraction occurs whenever light crosses a boundary between two different media — air to glass, air to water, water to glass, etc.
Mistake 4: Forgetting lateral inversion when describing mirror images. Mirror images are laterally inverted (left-right swapped). This is separate from being inverted top-to-bottom (which plane mirrors do NOT do — that’s for other mirror types).
Mistake 5: Saying a shadow is always the same shape as the object. Shadow shape depends on both the shape of the object AND the position/size of the light source. A circular ball can cast an elliptical shadow depending on angle.
Practice Questions
Q1. Name two luminous and two non-luminous objects from everyday life.
Luminous: Sun, electric bulb (or candle, torch, firefly — any two). Non-luminous: Moon, book (or table, chair, a planet — any two).
Q2. A ray of light strikes a plane mirror at an angle of 40° to the normal. What is the angle of reflection? What is the angle between the incident ray and the reflected ray?
Angle of reflection = 40° (equal to angle of incidence).
Angle between incident ray and reflected ray = 40° + 40° = 80°.
Q3. Why does a swimming pool always appear shallower than it actually is?
Light from the bottom of the pool travels from water (denser medium) to air (less dense medium). At the water surface, the light bends away from the normal (refraction). When this refracted light enters our eyes, our brain traces rays back in a straight line — the apparent intersection point is higher (shallower) than the actual position of the pool bottom. Hence the pool appears shallower.
Q4. What type of reflection allows us to read a book? Explain why the same type of reflection happens from a book but not from a mirror.
Diffuse (irregular) reflection. A book’s surface is rough at the microscopic level — different tiny portions of the surface are inclined at different angles. Parallel light rays hitting these different portions reflect in different directions. So light scatters in all directions, allowing us to see the book from any position.
A mirror’s surface is very smooth. All portions are at the same angle, so parallel rays reflect as parallel rays — this is regular (specular) reflection, which forms clear images but only lets you see the source from specific angles.
Q5. List the colours in the visible spectrum in order from least bent to most bent when white light passes through a prism.
Red, Orange, Yellow, Green, Blue, Indigo, Violet (ROYGBIV).
Red is bent least; Violet is bent most.
Q6. A person stands 4 m from a plane mirror. They walk towards the mirror at 1 m/s. How fast is the person approaching their own image?
The image is always as far behind the mirror as the person is in front. As the person walks 1 m/s toward the mirror, the image also moves 1 m/s toward the mirror (from behind).
So person and image approach each other at 1 + 1 = 2 m/s.
Q7. Explain why a pinhole camera forms an inverted image.
Light travels in straight lines (rectilinear propagation). Light from the top of the object passes through the pinhole in a straight line and hits the bottom of the screen. Light from the bottom of the object passes through the pinhole and hits the top of the screen. Since rays cross at the pinhole, the image formed is upside-down (inverted) and also laterally reversed.
Q8. Give one use each for opaque, translucent, and transparent materials in daily life.
- Opaque: Walls of a room (privacy, no light passes through)
- Translucent: Frosted glass in bathrooms (allows light but gives privacy)
- Transparent: Eyeglasses/spectacles, glass windows (clear vision needed)
FAQs
Why do we see our reflection in still water but not in a flowing river?
Still water has a smooth, flat surface — this gives regular (specular) reflection, forming a clear image. Flowing water has a rippled, uneven surface — this gives diffuse reflection, scattering light in all directions and preventing a clear image from forming.
Why is a rainbow always seen opposite to the Sun?
Raindrops act as tiny prisms. Sunlight enters each raindrop, undergoes refraction (splitting into colours), reflects off the back of the drop, and refracts again on exiting. This process sends the dispersed light back toward the observer — but only when the observer faces away from the Sun. So we always see a rainbow in the direction opposite to the Sun.
Can light travel through a vacuum?
Yes — and in fact, light travels fastest in a vacuum (about 3 × 10⁸ m/s). Light from the Sun reaches us through the vacuum of space. Sound cannot travel through vacuum, but light can.
Why does a glass of water make a white light on the wall look coloured?
A cylindrical glass of water acts somewhat like a prism. When white light passes through it, different colours (wavelengths) bend by different amounts, separating into the visible spectrum. The curved surface of the glass enhances this effect.
Why does the sky appear blue?
Sunlight (white light) enters Earth’s atmosphere, which has tiny particles (gas molecules). These particles scatter shorter wavelengths (blue, violet) much more than longer wavelengths (red, orange). Blue light scatters in all directions across the sky, so when we look up anywhere in the sky, we see scattered blue light. (For Class 7-8, this is beyond the syllabus, but knowing it helps connect concepts.)
What is the difference between a real image and a virtual image?
A real image is formed where light rays actually meet — you can project it on a screen (like a pinhole camera’s image). A virtual image is formed where rays only appear to meet (the backward extensions of reflected/refracted rays) — you cannot project it on a screen. A plane mirror forms a virtual image.
Why does a spoon look bent when placed in a glass of water?
The part of the spoon in water sends light to our eyes through water-to-air interface, where refraction bends it. The part above water sends light directly through air. Our brain sees the light from both parts but assumes it all traveled in straight lines — so the underwater part appears displaced/bent from where it actually is.
How does a periscope work?
A periscope has two plane mirrors fixed at 45° to the vertical, one at the top and one at the bottom, facing each other. Light from above hits the top mirror, reflects downward at 90°, travels to the bottom mirror, and reflects again into the observer’s eyes. The two reflections mean the final image is erect (right-side up). Submarines and soldiers in trenches use periscopes to see over obstacles without being seen.