Light: Brightness and Colour

Sources of light

• Luminous objects make their own light: the Sun, a candle flame, a light bulb, a phone screen.
• Non-luminous objects do not make light. We see them because they reflect light into our eyes: the Moon, a book, your friend's face.

Light travels in straight lines called rays. That is why shadows have sharp edges and why you cannot see round corners. Light travels at about $3\times 10^8$ m/s, the fastest speed in the universe.

Brightness

Brightness depends on how much light energy reaches your eye each second. Two main things affect it:

1. The power of the source. A 10 W LED is brighter than a 3 W one. For a light wave, a bigger amplitude means more energy, so brighter light.
2. The distance from the source. The further away, the dimmer it looks, because the light spreads out over a bigger area.

Brightness is also why stars look so different. A huge, powerful star far away can look dimmer than a small star nearby.

White light and the spectrum

In 1666, Isaac Newton shone sunlight through a glass prism. White light came in. A rainbow came out. White light is actually a mixture of all the colours.

The colours of the visible spectrum, in order:

Red, Orange, Yellow, Green, Blue, Indigo, Violet (remember ROY G BIV)

Colour depends on wavelength (and frequency). Red light has the longest wavelength and lowest frequency. Violet has the shortest wavelength and highest frequency.

A prism splits white light because each colour refracts by a slightly different amount. Violet bends the most, red the least. This is dispersion. Raindrops do the same, which makes rainbows.

Why objects have colour

An object's colour depends on which colours it reflects and which it absorbs.

• A red shirt reflects red light and absorbs all the other colours.
• A white object reflects all colours.
• A black object absorbs all colours (which is why black clothes get hot in the sun).

Back to Chloe's stage. Under green light, the red scarf has no red light to reflect. It absorbs the green. Nothing comes back, so it looks black.

Coloured filters

A filter lets its own colour through and absorbs the rest.

• A red filter lets red light pass and blocks everything else.
• White light through a red filter comes out red.
• Red light through a blue filter: nothing comes out (black).

Mixing coloured light

The primary colours of light are red, green and blue (RGB). Every screen you look at makes all its colours from tiny red, green and blue dots.

This is additive mixing. It is different from mixing paints, where the primaries are different and mixing everything gives a muddy dark colour.

Quick summary

• Luminous objects make light. Non-luminous objects reflect it.
• Brightness depends on source power (amplitude) and distance.
• Inverse square law: double the distance, one quarter the brightness.
• White light is a mix of all colours: ROY G BIV. Red has the longest wavelength, violet the shortest.
• Objects reflect their own colour and absorb the others. Filters let their own colour through.
• Primary colours of light: red, green, blue. All three together make white.

Quiz