Colour & Reflection of Light

Nature of Light and Colour

White light is made up of a mixture of different colours, each with its own wavelength. When white light passes through a prism or raindrops, it splits into a spectrum of colours. This process is called dispersion.

The visible spectrum consists of seven main colours, often remembered by the mnemonic ROYGBIV: Red, Orange, Yellow, Green, Blue, Indigo, Violet. Each colour corresponds to a different wavelength of light:

• Red has the longest wavelength (~700 nm)
• Violet has the shortest wavelength (~400 nm)

Note: Some GCSE specifications simplify the visible spectrum by omitting Indigo, using six main colours instead.

Colour is perceived because objects reflect certain wavelengths of light and absorb others. The wavelength of the reflected light determines the colour we see.

Dispersion occurs because different wavelengths of light refract (bend) by different amounts when passing through materials like glass or water. Shorter wavelengths (blue/violet) bend more than longer wavelengths (red), causing the white light to spread out into a spectrum.

For instance, when sunlight passes through a glass prism, the white light separates into a rainbow of colours due to dispersion.

Example: White light passes through a prism and spreads out into a spectrum. The red light bends the least, while violet bends the most, causing the colours to fan out.

Reflection of Light

Reflection is the bouncing back of light rays when they hit a surface. The law of reflection states:

• The angle of incidence equals the angle of reflection.
• Both angles are measured from the normal, which is an imaginary line perpendicular to the surface.

There are two types of reflection:

• Specular reflection: Occurs on smooth, shiny surfaces like mirrors. The reflected rays are parallel, producing a clear image.
• Diffuse reflection: Occurs on rough surfaces like paper or walls. The reflected rays scatter in many directions, so no clear image is formed.

Plane mirrors produce virtual images that are:

• Upright
• Same size as the object
• Laterally inverted (left and right reversed)
• Located the same distance behind the mirror as the object is in front

Example: If a ray of light hits a mirror at an angle of 306 to the normal, it will reflect off at 306 on the other side of the normal.

Colour and Reflection

Objects appear coloured because they absorb some wavelengths of light and reflect others. The colour we see is the colour of the light reflected.

For example, a red apple appears red because it reflects red light and absorbs other colours.

Black objects absorb most of the light that hits them and reflect very little, so they appear black. White objects reflect most of the light across all visible wavelengths, so they appear white.

Coloured filters work by absorbing certain colours of light and only allowing others to pass through. For example, a red filter absorbs all colours except red, which it transmits.

If white light passes through a coloured filter, the light that emerges is the colour of the filter. This affects the colour of objects viewed through the filter.

Example: If a green object is viewed through a red filter, it will appear black because the red filter blocks green light, so no light is reflected to the eye.