Visible Light — Grade 8 Natural Sciences

Properties of light

Light is a form of energy that is emitted from luminous objects such as the Sun, light bulbs, and flames. Key properties:

Light travels in straight lines (rays)
Light travels at 300 000 km/s through empty space
It does not need a medium (it can travel through a vacuum)
It is transferred by radiation

The spectrum of visible light

White light is not a single colour — it is a mixture of all the colours of the spectrum. When white light passes through a triangular prism, it is dispersed (split) into its component colours, from lowest frequency (longest wavelength) to highest:

Violet · Indigo · Blue · Green · Yellow · Orange · Red (mnemonic: VIBGYOR)

A rainbow forms when sunlight is refracted and dispersed by water droplets in the atmosphere — the same principle as a prism.

The colours start separating the moment they enter the glass and keep spreading as they cross it. When they reach the second face they bend again — away from the normal this time — and spread apart much more sharply. Violet bends most at both faces, red least, so white light fans out into the spectrum.

Remember: Violet/indigo/blue light has the highest frequency (shortest wavelength). Red/orange light has the lowest frequency (longest wavelength). Frequency details are NOT required at this level.

Opaque and transparent substances

Opaque

Light cannot pass through. It is absorbed or reflected. Opaque objects cast shadows. Examples: metal, brick, wood, cardboard.

Transparent

Most light passes through. Some is absorbed and some reflected, but most transmits. Examples: glass, clear plastic, clean water.

Absorption and colour

When white light hits a coloured surface, some frequencies (colours) are absorbed and others are reflected. The colours that reach your eye determine what colour you see:

A red wall absorbs violet, indigo, blue, green, yellow, orange — and reflects red
A black object absorbs ALL frequencies — no light is reflected, so it looks black
A white object reflects ALL frequencies — all colours reach the eye, so it looks white

Reflection of light

Light bounces off surfaces. The law of reflection states: the angle of incidence = the angle of reflection (both measured from the normal — an imaginary line perpendicular to the surface).

Smooth surfaces (mirrors): all light reflects in the same direction → clear image
Rough surfaces (crumpled foil, wall): reflected light scatters in many directions → no clear image (diffuse reflection)

Refraction of light

When light travels from one medium (e.g. air) into another (e.g. glass or water), it changes speed. If it enters at an angle, it also changes direction. This is called refraction:

Light entering a denser medium (air → glass) bends towards the normal (slows down)
Light leaving a denser medium (glass → air) bends away from the normal (speeds up)
A triangular prism refracts light twice — once entering and once leaving — dispersing it into the spectrum
A lens refracts light to focus it to a point

You can see refraction by placing a pencil in a glass of water — the pencil appears bent at the water surface.

Write your answers in the spaces below. Use full sentences.

1. List the colours of the visible spectrum in order from highest frequency to lowest frequency. What mnemonic can help you remember the order?

2. Explain the difference between an opaque and a transparent substance. Give two examples of each and explain what happens to light when it hits these substances.

3. A red car is parked in sunlight. Explain in terms of light absorption and reflection why the car appears red.

4. State the law of reflection and draw a ray diagram to illustrate it. Label the normal, angle of incidence, and angle of reflection.

5. What is refraction? A ray of light travels from air into a glass block at an angle. In which direction does it bend? What happens when it exits the glass back into air?

6. Explain how a triangular prism disperses white light into the visible spectrum. Why does violet light bend more than red light?

7. A learner uses a ray box and a plane mirror to test the law of reflection. She measures the angle of incidence and the angle of reflection (both from the normal) for four different trials.

Trial	Angle of incidence	Angle of reflection
1	20°	20°
2	35°	35°
3	50°	48°
4	65°	65°

(a) Based on Trials 1, 2 and 4, state the law of reflection in your own words. (2)
(b) Which trial does NOT fit this pattern? (1)
(c) Suggest ONE possible reason for the unexpected result in that trial (think about possible sources of experimental error, not a new law of physics). (1)