Physics p7

Behaviour of waves

Light, water and sound waves can be

Refracted and diffracted

Refraction – when waves cross a boundary between one medium and another, the frequency remains the same but there is a change in wavelength. This leads to a change in wave speed, which causes the wave to change direction.

Diffraction – when waves move through a narrow gap or obstacle, they spread out from the edges. This is called diffraction. Diffraction is most obvious when:

·         The size of the gap is similar to, or smaller than the wavelength of the wave.

·         The waves that pass obstacles have long wave lengths.

Light waves need a very small gap to be diffracted.

The fact that light and sound can be diffracted provides evidence of their wave nature.

Radiation is diffracted by the aperture (gap/opening) of a telescope. To produce sharp images, the aperture must be very much larger than the wavelength of the radiation detected by the telescope.

Refraction of white light

The colours that make up white light are refracted by different amounts as they pass through a prism:

·         Red light is refracted the least

·         Violet light is refracted the most

This refraction occurs because the colours that make up white light have different frequencies and different wavelengths.

A spectrum can also be produced when white light passes through a diffraction grating.

Convex lenses

A convex (or converging) lens bends rays of light inwards as they pass through the lens. If the rays of light entering the lens are parallel, the rays will be bought to a focus at a focal point. This is due to refraction.

The curvature of a lens, the more powerful it will be. So, if two lenses are made of the same material, a highly curved lens will be more powerful than a flatter lens.

You can calculate the power of a lens using this formula:

Power (dioptres) = 1/ Focal length (metres)

The focal length is the distance between the focal point and the lens.

Ray diagrams

Ray diagrams show how the image of an object would be formed.

You may be required to interpret ray diagrams for convex/ converging lenses gathering light from distant point sources (stars).

You need to be able to draw ray diagrams for the formation of real images from a

·         Distant point source (stars)

·         Distant extended source (planets, galaxies and moons in our solar system). The image produced will be inverted and smaller.

1.        Draw a ray line that runs from the top of the object parallel to the principal axis. At the middle of the lens bend this ray inwards so it passes through the focal point.

2.       Draw a second ray that runs from the top of the object straight through the centre of the lens as it crosses the principal axis.

3.       Draw a third ray that runs from the top of the object through the focal point on…