Electromagnetic Spectrum

All waves can be reflected or refracted (change speed when entering a new medium). This how we know EM spectrum are also waves.

All EM-waves are transverse. And they all travel with the same speed in vacuum (3x10⁸ m/s), and they do not need a medium to transfer energy. 

We should remember the order of the EM waves.

We should also remember some of their wavelength:

The higher the frequency of the EM wave, the more energy it has (and more dangerous it is).

P4.2-1 Applications of EM waves

Radio waves: TV and radio communications. Transmitter sends out the signal, and receiver receives it! The transmitters are put on top of high towers not to be stopped by large buildings and other obstructions. 

Higher tier only: radio waves can be produced by oscillations in electrical circuits. In radio transmitters current changes constantly and this changes the electric field produced by the current which in turn produces radio signals. 

In receivers the radio wave induces oscillations in the electric circuit, which can be converted to sound in a radio for example.

Radio waves are refracted in ionosphere, where the frequency determines the amount of refraction. 

The lower the frequency of radio wave, the higher the refraction. 

Microwaves have higher frequency than radio waves, and hence are not refracted by ionosphere, so they can be used for satellite communication. 

Microwave: transmit mobile phone signals, heating food and water. Microwaves used for cooking have higher frequency than mobile phones. Microwave vibrate molecules of water and fat in food and heat it up. 

Infrared: any warm object emits infrared radiation. Traditionally we cook food by infrared, which heats the surface of food, and then heat is transferred deeper by conduction. TV remote, and thermal imaging cameras also use infrared. 

Ultraviolet: is emitted from very hot objects (4000 ^oC and above) like the Sun. UV is used in fluorescent lights, tanning beds, disinfecting theatres in hospitals, security strip in banknotes.

Gamma-rays: kill cancer cells, or reduce size of a tumour (radiotherapy), medical imaging with gamma tracer and camera. 

X-rays: imaging bones, computerised tomography (CT) scans used to image body’s soft tissue, radio therapy for cancer treatment, security in airports. 

Higher tier: both gamma and x-rays have high frequency and energy, penetrative, and highly ionising, which can cause mutation of DNA and create cancerous cells. But they can be used to kill cancerous cells as well! Gamma-ray is produced from nucleus of radioactive atom, but X-ray is produced by colliding high speed electrons with metals. 

P4.2-2 Reflection and refraction

Reflection: 

When a wave reaches the boundary between two materials, it may be reflected (bounce back), transmitted, or absorbed; and these all may happen at the same time, this depends on two things: wavelength and the medium the wave enters. Like when you look out the window: you see what is outside (light is transmitted), but what you see is not as clear as if you were outside (as some light is absorbed by the glass), and you also can see a little reflection of yourself in the glass too (reflection!).

Higher tier: If surface of a reflector is flat and smooth (e.g. mirror), all light rays will be reflected with the same angle that came into the surface. In this case the reflection is called specular and an image can be seen from this reflection. If the surface is rough (e.g. clothes) it causes a diffuse reflection and an image cannot be seen from this. 

Refraction: happens when a wave is transmitted from one medium to another, it changes speed and may change direction. The shorter the wave length, the more refraction happens.

Higher tier: A wavefront is a line we draw to show all the points that move together on a wave. The shadows that you see on the screen below the ripple tank are like that! 

When water wave in the ripple tank goes to a shallower part, it is refracted so it changes speed and hence direction. Wave front hits the boundary at point A first, and this part of the wave slows down first, and point B is the last place to hit the boundary. This is why the wave changes direction (provided that the wave hits the boundary at an angle in the first place!). If the wave hits the boundary at right angles, it slows down but does not change direction!

Note: in diffraction wave speed and wavelength change, but not the frequency! (v = λ f).

When light slows down it gets closer to the normal. The normal is line we draw perpendicular to the boundary between the two materials (in image below i₂ < i₁).