Physics - Light Waves and Refraction

What is the equation that links velocity, frequency and wavelength?

v=f x λ

What type of wave is light?

Electromagnetic wave - transverse wave

What do waves do?

Transfers energy and information without transferring matter.

What is the speed of light in air (or a vacuum)?

3x10^8m/s

What is a longitudinal wave?

When the vibration is transmitted parallel to the direction of energy transfer.

What is a transverse wave?

The vibration is transmitted perpendicular to the direction of energy transfer.

How do you calculate the frequency of a wave?

Frequency (Hz) = 1 ÷ time period (s)

What is the equation for the refractive index?

n=sin(i)/sin(r)

How can you calculate the refractive index using speed of light?

n = speed of light in vacuum (air)/speed of light in medium

When a light ray goes from a less dense medium to a denser medium, which way does it bend?

Towards the normal

What is parallel with the incident ray?

The emergent ray which bends away from the normal.

When does dispersion occur?

When a prism splits the colour of white light into the spectrum

What has colours have the greatest and lowest refractive index?

Violet has the greatest (shorter wavelength) and red has the lowest (longest wavelength). (ROYGBIV)

What is the time period of a wave?

The time it takes for one complete wave to pass a point

Which direction do waves carry and transfer energy in?

The direction they are travelling

What are wavefronts?

Imaginary planes that cut across all the waves, connecting the points on adjacent waves which are vibrating together.

How do we see things?

Light bounces off things into our eyes.

What is it called when light reflects from an uneven surface?

Diffuse reflection - the light reflects off at all different angles

What is it called when light reflects from an even surface?

Clear reflection - the light reflects at the same angle.

What is the law of reflection?

Angle of incidence = angle of reflection

What is the normal?

An imaginary line that is perpendicular to the surface at the point of incidence.

What is the angle of incidence?

The angle between the incoming wave and the normal

What is the angle of reflection?

The angle between the reflected wave and the normal

When can waves travel at different speeds?

When travelling through mediums which have different densities

What happens when a wave hits a different medium at an angle?

The bit that hits the denser layer first slows down while the other part carries on at the first, faster speed so the wave changes direction.

What does a ray diagram show?

The path that a wave travels

How do you experiment with refraction?

Shine a light ray at angle into a block on a piece of paper. Trace the incident, emergent and refracted rays on the paper.

Why does white light refract into a rainbow when it is shone through a prism?

White light is made up of all the visible frequencies of light. They have different wavelengths so they refract by different amounts so it disperses into the different colours as it enters the prism.

What is the refractive index?

Tells you how much the speed of light is reduced compared to travelling in a vacuum.

How do you find the refractive index of a glass block?

Draw around the rectangular glass block and shine a ray of light through it at an angle. Trace the incident, emergent, and refracted ray onto the paper. Draw in the normal and use a protractor to measure the angle of incidence and refraction and use Snell's law to calculate.

Which material will bend more towards the normal, n=1.5 or n=1.3?

n=1.5 because it is slower

What is the critical angle?

The angle of incidence when the angle of refraction is 90 degrees

What comes after the critical angle?

Total internal reflection - no light leaves the medium

Describe an experiment to test for total internal reflection...

Aim a light ray at the curved edge of a semi circle on a piece of paper. Shine the light ray at the same point on the block every time, roughly in the middle of the flat edge. Keep increasing the angle of incidence until you find the critical angle. Then go beyond this to see total internal reflection.

How can you find the critical angle?

sin C (critical angle) = 1/n

The higher the refractive index...

the lower the critical angle

What do optical fibres contain?

Made of plastic or glass with a central core surround by cladding with a lower refractive index.

Why is the core fibre in fibre optics so narrow?

So light signals passing through it always hit the core-cladding boundary at angles higher than C so light is always totally internally reflected.

What does total internal reflection allow us to do?

Use prisms to see objects that aren't in our direct line of sight - periscopes.

When does total internal reflection occur?

Can only occur when travelling from a dense medium to a less dense medium e.g. from water to air

What are the uses of TIR?

Prisms in periscopes, reflectors, and optical fibres used for communication

How do reflectors work?

Made up of many small prisms arranged so that light undergoes total internal reflection twice, resulting in the light being returned in the direction it originally came from.

How do optical fibres transmit information?

Using visible light or IR radiation. The light cannot escape from the fibre so it is continually reflected internally by the fibre.

Why are optical fibres better than microwaves and radio waves?

Can carry far more information due to the higher frequency of visible light and IRR; more secure because signals stay within the fibre.

What is refraction?

The bending of light when it passes from one medium to another

What is refraction caused by?

A change in the speed of light

Why does refractive index have no unit?

n=sin(i)/sin(r). a sine is just a number so it doesn't have a unit.

Why is TIR more likely with a higher refractive index?

Higher refractive index means smaller critical angle. TIR occurs when the angle of incidence is greater than the critical angle. Therefore, there is a greater range of angles for TIR to occur.

Why does light refract towards the normal when travelling from less dense to dense medium?

The light slows down in the glass so the refractive index is greater and it refracts more.

What is total internal reflection?

The light ray reflects inside the material and all of the light is reflected.

Explain what happens to light inside optical fibres?

Light reflects off the surface of the fibre with an angle of incidence greater than the critical angle because light travels slower in glass.

Explain how you can make the experiment to find the refractive index more accurate?

Use a protractor; vary the angle of incidence to obtain a range of values; use n=sin(i)/sin(r) to calculate; plot a graph of sin(i) against sin(r).