Light waves - Paper 1 - flashcards

What is the Law of Reflection?

Angle of incidence = Angle of reflection

Difference between speed of light vs speed of sound waves in dense mediums?

light waves travel more slowly in dense mediums, while sound waves speed up.

What is diffuse reflection?

When light reflects off an uneven surface at all different angles. E.g. light on a piece of paper.

What is a clear reflection?

When light reflection off a smooth surface and is all reflected at the same angle.

What is the normal?

An imaginary line that is perpendicular to the surface at the point of incidence. It is usually shown as a dotted line.

What is specular reflection?

The type of reflection that happens with a flat mirror.

The image in the mirror is…

* Laterally inverted
* At the same height as the object
* As far behind the mirror as the object is in front of it
* Virtual

What are virtual images?

Images formed when light rays bounce off an object onto a mirror and diverge, so the light from the object appears to be coming from a completely different place. This happens with a mirror, and the image appears to be behind the mirror.

What is refraction and why does it occur?

Refraction is the change in direction of a wave.

* This happens when when travel through mediums of different densities, because the change in density cause the waves to travel at different speeds.
* Light waves travel slower in denser substances.
* Refraction only occurs when waves meet the boundary of the surface at an angle.
* If the wave hits the boundary face-on, it slows down but travels through the material in the same direction.

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What happens to light as it passes from into a denser medium?

The light wave bends towards the normal, because it slows down.

Why do triangular prisms disperse white light?

Because different wavelengths of light refract by different amounts, so white (a mixture of all visible frequencies) disperses into different colours as it enters a prism.

What is a refractive index?

It is a measure of the change in the speed of light as it passes from a vacuum (or air) into the material.

A value which tells you how fast light travels in a particular material.

Discuss refractive indexes.

* Light slows down a lot in glass, so the refractive index of glass is high (around 1.5).
* The refractive index of water is lower - (around 1.33) as light doesn’t slow as much in water as in glass.
* The speed of light in air is about the same as in a vacuum, so the refractive index of air is near enough to 1.00.

What happens to the angle of refraction as the angle of incidence increases?

The angle of refraction increases too.

* However, this relationship isn’t directly proportional.

What is the critical angle?

When the refracted ray is 90 degrees. At this angle, light is refracted along the boundary.

What happens to light at an angle greater than the critical angle?

The light is totally internally reflected - no light leaves the medium.

Why are diamonds so sparkly/

Because they have a very low critical angle, so light at even small angles is internally reflected.

What is the critical angle for water?

49 degrees

Discuss optical fibres.

* Made of plastic or glass
* Very little light is absorbed by the glass.
* They consist of a central core surrounded by cladding with a lower refractive index.
* The core of the fibre is so narrow that light signals passing through it always hit the core-cladding boundary at angles higher than the critical angle -- so light is always totally internally reflected.

What is their role?

* Used for fibre optic broadband - to send computer information coded as pulses of light along underground optical fibres.
* Fibreglass is much cheaper than copper wire for carrying electrical signals.
* Used as an endoscope: guides light into a patient and guides the reflected light back out to give an image.