## Electromagnetic Waves

Electromagnetic waves are transverse waves that transfer energy  from the source of the waves to an absorber.

Electromagnetic waves form a continuous spectrum and all types of  electromagnetic wave travel at the same velocity through a vacuum  (space) or air.  That velocity is 300 000 000 m/s.

The waves that form the electromagnetic spectrum are grouped in  terms of their wavelength and their frequency. Going from long to  short wavelength (or from low to high frequency) the groups are:  radio, microwave, infrared, visible light (red to violet), ultraviolet,  X-rays and gamma rays.

The highest frequency waves (gamma) have the highest energy and shortest wavelength.

Question: Explain why it is important that the Earth’s atmosphere absorbs gamma rays emitted by the Sun.

Question: A new technology has been developed to charge small electrical devices from the radio waves in the air around us.
Explain how radio waves could charge a small electrical device.

## Refraction & Reflection of EM Waves

Different substances may absorb, transmit, refract or reflect  electromagnetic waves in ways that vary with wavelength.

Refraction is due to the  difference in velocity of the waves in different substances. Visible light is refracted and reflected at a boundary with glass

Wave front diagrams can be used to  explain refraction in terms of the change of speed that happens  when a wave travels from one medium to a different medium. Not all of the wave enters the new medium at the same time, so the first part that enter will slow down more than the part not in the medium. His causes the wave to change direction.

Question: Wave front diagrams are used to explain why light refracts when it passes from air into glass.
Figure 1

(a)  Explain why the light refracts as it passes from air into glass. (3 marks)

## Black Body Radiation (Triple Physics Only)

All bodies (objects), no matter what temperature, emit and absorb infrared radiation. The hotter the body, the more infrared radiation it radiates in a given time.

A perfect black body is an object that absorbs all of the radiation that is directed towards it.

Learn this definition

A black body does not reflect or transmit any radiation. Since a good absorber is also a good emitter, a perfect black body would be the best possible emitter.

All bodies (objects) emit radiation

A body at constant temperature is absorbing radiation at the same rate as it is emitting radiation. The temperature of a body increases when the body absorbs radiation faster than it emits radiation.

The temperature of a body is related to the balance between incoming radiation absorbed and radiation emitted.

The temperature of the Earth depends on many factors including: the rates of absorption and emission of radiation, reflection of radiation into space. If the Earth receives more radiation than is reflected/emitted back into space, the Earth will heat up.

Question: Albedo is a measure of the amount of solar radiation reflected by an object compared to the total solar radiation incident on the object.
A perfect reflector has an Albedo value of 1.0
A perfect absorber has an Albedo value of 0.0
What is the Albedo value of a perfect black body?