Light

Learn it by playing

Answer these questions to earn energy, then fish and explore. No account needed.

For teachers: ready-to-use lesson slides, revision notes, diagrams for Light (Physics, CIE) — use them in your lesson, or run the topic as a live class game.

Notes

Reflection of Light

A **normal** is drawn at right angles to the boundary between two media.
Angles are measured between the ray and the normal: **angle of incidence (i)** and **angle of reflection (r)**.
**Law of reflection**: angle of incidence = angle of reflection (i = r).
In a plane mirror, the image is **virtual**, **same size**, **same distance behind** the mirror as the object is in front, and **laterally inverted**.
Ray diagrams show incident and reflected rays with arrows indicating direction.

Investigating Reflection

**Aim**: To investigate reflection by a plane mirror.
**Independent variable**: angle of incidence; **dependent variable**: angle of reflection.
Use a ray box, protractor, paper, pencil, ruler, and plane mirror.
Mark incident and reflected ray positions, then measure angles with a protractor.
Repeat for different angles of incidence to verify i = r.

Refraction of Light

Refraction occurs when light passes between two transparent media of different **optical density**.
From less dense to more dense (e.g., air to glass): light bends **towards** the normal.
From more dense to less dense (e.g., glass to air): light bends **away** from the normal.
If light travels along the normal, it does **not bend**.
Frequency remains constant; speed and wavelength change.

Refractive Index

Refractive index n = speed of light in vacuum / speed of light in material.
Also n = sin i / sin r (Snell's law).
n is always > 1 and has no units.
Higher n means more optically dense (e.g., diamond n ≈ 2.4, glass n ≈ 1.5).

Total Internal Reflection

**Total internal reflection (TIR)** occurs when light travels from a denser to a less dense medium and the angle of incidence exceeds the **critical angle (c)**.
Critical angle: angle of incidence for which angle of refraction = 90°.
Relationship: sin c = 1/n.
Higher n gives smaller critical angle.
Uses: optical fibres (communications, endoscopes), prisms in periscopes, binoculars.

Ray Diagrams for Lenses

**Converging (convex) lens**: brings parallel rays to a focus at the **principal focus (f)**.
**Diverging (concave) lens**: makes parallel rays diverge; virtual focus on same side as incoming light.
**Focal length**: distance from lens centre to principal focus.
Three key rays: through optical centre (undeviated), parallel to axis (through focus), through focus (emerges parallel).

Real & Virtual Images

**Real image**: formed where rays **converge**; can be projected on a screen; always **inverted**.
**Virtual image**: formed where rays **appear to diverge**; cannot be projected; always **upright**.
Converging lens can produce real or virtual images depending on object position.
Diverging lens always produces a **virtual, upright, diminished** image.

Correcting Sight & Dispersion

**Long-sightedness**: image forms behind retina; corrected with **converging (convex) lens**.
**Short-sightedness**: image forms in front of retina; corrected with **diverging (concave) lens**.
**Dispersion**: white light splits into colours when refracted by a prism; violet bends most, red least.
Visible spectrum: red, orange, yellow, green, blue, indigo, violet (longest to shortest wavelength).
**Monochromatic light**: single frequency/colour (e.g., laser).

Reflection of light showing the normal, angle of incidence (i) and angle of reflection (r).

Refraction of light from air into glass: light bends towards the normal.

Practice questions

Free preview — 8 of 40 questions. Sign up to see them all.

1.What is the law of reflection?
Easy
- AAngle of incidence equals angle of reflection
- BAngle of incidence equals angle of refraction
- CAngle of reflection is twice the angle of incidence
- DAngle of incidence is greater than angle of reflection
2.Which type of image cannot be projected onto a screen?
Easy
- AReal image
- BVirtual image
- CInverted image
- DMagnified image
3.When light passes from air into glass, it bends towards the normal. What happens to the speed of light?
Easy
- AIt increases
- BIt decreases
- CIt stays the same
- DIt becomes zero
4.A ray of light enters a glass block of refractive index 1.5 at an angle of incidence of 30°. What is the angle of refraction? (sin 30° = 0.5)
Medium
- A19.5°
- B30°
- C45°
- D9.7°
5.What is the critical angle for a material with refractive index 1.5?
Medium
- A41.8°
- B25°
- C90°
- D30°
6.Which of the following is a condition for total internal reflection to occur?
Medium
- ALight travels from a less dense to a more dense medium
- BThe angle of incidence is less than the critical angle
- CLight travels from a more dense to a less dense medium and the angle of incidence is greater than the critical angle
- DThe angle of incidence equals the angle of refraction
7.A converging lens forms a real image of an object placed at 2F. Where is the image formed?
Hard
- AAt F
- BAt 2F on the opposite side
- CBeyond 2F
- DBetween F and 2F
8.A student measures the angle of incidence as 40° and the angle of refraction as 25°. What is the refractive index of the medium? (sin 40° = 0.643, sin 25° = 0.423)
Hard
- A1.52
- B0.66
- C1.0
- D2.0

Slides

Reflection of Light

In optics, a normal line is drawn at right angles to the boundary between two media.
The angle of incidence (i) is the angle between the incident ray and the normal.
The angle of reflection (r) is the angle between the reflected ray and the normal.
The law of reflection states: angle of incidence = angle of reflection (i = r).

Which statement about reflection is correct?

The angle of incidence is always greater than the angle of reflection.
The angle of incidence equals the angle of reflection.
The angle of incidence is measured from the mirror surface.
The normal line is parallel to the mirror.

Refraction of Light

Refraction occurs when light passes across a boundary between two transparent materials.
When light enters a denser medium (e.g., air to glass), it bends towards the normal.
When light enters a less dense medium (e.g., glass to air), it bends away from the normal.
If light travels along the normal, it does not bend.

When light passes from air into glass, it bends ____ the normal. When it passes from glass into air, it bends ____ the normal.

Refractive Index

The refractive index (n) measures how much light bends when entering a material.
It is defined as n = sin i / sin r, where i is the angle of incidence and r is the angle of refraction.
Refractive index has no units and is always greater than 1.
A higher refractive index means light bends more.

A ray of light enters a glass block with refractive index 1.5. If the angle of incidence is 30°, what is the angle of refraction? (sin 30° = 0.5)

19.5°
30°
45°
60°

Total Internal Reflection

Total internal reflection (TIR) occurs when light travels from a denser to a less dense medium and the angle of incidence is greater than the critical angle.
The critical angle (c) is the angle of incidence that produces an angle of refraction of 90°.
The critical angle is related to refractive index by sin c = 1/n.
TIR is used in optical fibres, periscopes, and endoscopes.

Which of these are true about total internal reflection?

TIR occurs when light travels from a less dense to a denser medium.
The critical angle is the angle of incidence for which the angle of refraction is 90°.
Optical fibres use TIR to transmit light.
A larger refractive index gives a larger critical angle.

Real and Virtual Images

A real image is formed where light rays converge and can be projected onto a screen. It is always inverted.
A virtual image is formed where light rays appear to diverge from; it cannot be projected and is always upright.
Plane mirrors produce virtual images that are the same size and same distance behind the mirror as the object is in front.

Which of the following is a property of a virtual image formed by a plane mirror?

It can be projected onto a screen.
It is inverted.
It is the same size as the object.
It is formed in front of the mirror.

Converging Lenses

A converging (convex) lens brings parallel rays of light to a focus at the principal focus.
The distance from the lens to the principal focus is the focal length.
Ray diagrams use three rules: rays through the centre pass straight, rays parallel to the axis go through the focus, rays through the focus emerge parallel.

A converging lens is also called a ____ lens. It brings parallel rays to a ____.

Diverging Lenses

A diverging (concave) lens spreads out parallel rays so they appear to come from a virtual focus.
All images formed by a diverging lens are virtual, upright, and diminished.
The image is always on the same side of the lens as the object.

Which type of image is always produced by a diverging lens?

Real and inverted
Virtual and upright
Real and upright
Virtual and inverted

Correcting Sight

Long-sightedness (hyperopia) is corrected with a converging (convex) lens.
Short-sightedness (myopia) is corrected with a diverging (concave) lens.
Long-sighted eyes focus light behind the retina; converging lenses add extra convergence.
Short-sighted eyes focus light in front of the retina; diverging lenses spread the light before it enters the eye.

Which of these are correct?

A converging lens corrects short-sightedness.
A diverging lens corrects short-sightedness.
Long-sightedness is corrected with a concave lens.
Long-sightedness is corrected with a convex lens.

Dispersion of Light

White light can be split into a spectrum of colours by a prism.
This is called dispersion and occurs because different colours refract by different amounts.
Violet light is refracted most; red light is refracted least.
The visible spectrum colours in order: red, orange, yellow, green, blue, indigo, violet.

Which colour of light is refracted the most when passing through a prism?

Red
Green
Blue
Violet

Unlock all 40 questions, slides & more

Create a free account to see every question, the slides, flashcards and revision notes for this topic.

Past papers

Past-paper practice for this topic is coming soon.

🗂️ Coming soon