P1c(ii) A Spectrum of Waves

You will learn about:Electromagnetic Waves

How the behaviour of waves allows us to use them

www.PhysicsGCSE.co.uk

The Electromagnetic Spectrumwww.PhysicsGCSE.co.uk

Important facts:1. The Electromagnetic Spectrum is comprised of seven types of transverse wave.2. Wavelength decreases as frequency increases.3. Radio waves have the longest wavelength, lowest frequency and lowest energy.4. Gamma waves have the shortest wavelength, highest frequency and highest energy.5. All Electromagnetic waves travel in straight lines, they do not bend around corners e.g. you cannot see someone hiding around a corner.6. They can all be reflected.7. They can all be refracted e.g. when light passes from one medium to another at an angle to the boundary it changes direction. This can produce some strange effects.8. SOME Electromagnetic waves can be used for communication.

ReflectionWhen an Electromagnetic Wave meets a shiny surface it will be reflected. This surface is called a boundary.The non-hatched edge indicates the shiny side.A line drawn at a right angle to the boundary is called the Normal or N.The ray meeting the boundary is called the Incident Ray.The ray being reflected is called the Reflected Ray.The angle made between the Incident Ray and the Normal is called the Angle of Incidence or θi

The angle made between the Reflected Ray and the Normal is called the Angle of reflection or θr

www.PhysicsGCSE.co.uk

Normal

Incident Ray

Reflec

ted R

ay

Angle of Incidenceθi

Angle of Reflectionθr

BoundaryShiny side

BoundaryNon-Shiny side

REMEMBER: Always draw arrows on rays so you can indicate direction.Angles are ALWAYS drawn from the Normal, not the Boundary.

Law of Reflection:

The angle of Incidence = The angle of Reflection θi = θrContextualisation:

The light from the kitten reaches the boundary at an angle of 30° to the Normal.This means that the light will be reflected at 30° to the Normal.If you stand at any angle other than 30° to the Normal you will not see the complete reflected image of the kitten.See kitten

here!

30°

Light changing directionwww.PhysicsGCSE.co.uk

Even though light travels in straight lines it can be reflected around corners. Note that the mirrors are set at 45° angles.

Reflection

Refraction When light travels from air and through a pane of glass its frequency does not change. If it did the light would not be light anymore.

The glass is much more dense that the surrounding air. Therefore light slows down when it enters it. If it slows down and its frequency DOES NOT change then its wavelength has to.

You can see the wavelength is much shorter once the light has entered the glass.

Also note that when the light passes through the boundary the light changes direction too. This is refraction

Refraction causes objects to appear differently. You can see where the pencil should be at X.But because the light is travelling from a more dense medium the light has changed direction so it appears elsewhere, at Y. Your brain has been tricked.This causes a lot of problems for astronomers wanting to observe celestial objects.

During WW1 periscopes were used to safely see over trench walls. This meant the soldiers were less likely to be spotted.

Diffraction

www.PhysicsGCSE.co.uk Here you can see that when the water passes through the small gap the waves spread out in an arc. This is called diffraction.

If the distance between two waves, the wavelength, is equal to the separation distance of the gap then total diffraction occurs.

If the distance between two waves, the wavelength, is much less than the separation distance of the gap then partial diffraction occurs. Diffraction is less noticeable.

When waves intersect with one another interference patters can be observed.

A reflector telescope is an excellent choice to observe distance stars and planets.However, when the light enters the telescope some of it hits the Secondary mirror. This means the light spreads out around the secondary mirror before being reflected by the Primary mirror. This reduces the quality of the image that the observer sees.Diffraction can also occur at the edge of any lens e.g. microscope or telescope. You will notice a spiking effect when light diffracts.

Questions1. A ray of light strikes a flat reflective surface with an angle of

incidence of 10°. What is the angle of reflection?2. The angle between a non-curved mirror and ray of light is

20°. What is the angle of reflection?3. Which part of the electromagnetic spectrum has the longest

wavelength? 4. An aerial on a mobile phone is much shorter than the aerial

for a radio. Why is this?5. If you wanted a wave to be diffracted as well as possible how

would you achieve this?6. When light is being refracted through a more dense medium

what happens to its wave speed, frequency and wavelength?

www.PhysicsGCSE.co.uk

Questions1. A ray of light strikes a flat reflective surface with an angle of incidence of 10°.

What is the angle of reflection? 10° 2. The angle between a non-curved mirror and ray of light is 20°. What is the angle

of reflection? 70°3. Which part of the electromagnetic spectrum has the longest wavelength?

Radio 4. An aerial on a mobile phone is much shorter than the aerial for a radio. Why is

this? Radio waves have much longer wavelengths than the shorter radio (or micro) waves for mobile phones. So a much larger aerial is needed to receive them. Otherwise only part of the wave would be received.

5. If you wanted a wave to be diffracted as well as possible how would you achieve this? Ensure the wavelength of the wave is equal to the gap separation distance.

6. When light is being refracted through a more dense medium what happens to its wave speed, frequency and wavelength? Wave speed slows down because object is more dense ie there are more particles in the way so this slows down the wave. Frequency does not change. Wavelength reduces to compensate for the slower wave speed.

www.PhysicsGCSE.co.uk