Light Flashcards
What is light?
A type of energy, probably the fastest thing in the universe. Infrared and ultraviolet light are on the ends of visible light. It allows us to see. It can travel through a vacuum. The intensity of light is measured in lux.
How do you draw ray diagrams?
Use a pencil and ruler. No gaps in rays of light. Mark direction of light with a ruler. Draw normal at 90 degrees to surface and measure angles from the normal to the ray.
Why do you need a flat surface for clear reflection?
On a flat surface, light will reflect off at one angle so you will see a reflection. This is called specular reflection. On a rough surface light will be reflected at different angles, so the surface looks matt and you do not get a reflection. This is called diffuse reflection.
What is the law of reflection?
Angle of incidence = angle of reflection
What are virtual and real images?
Real images are formed when the light rays actually come together to form the image. The image can be captured on a screen because light rays meet at the place where the image seems to be e.g. the image forms on the eye’s retina. They are produced by converging lenses.
Virtual images are when the light rays from an object appear to be coming from a completely different place from where they actually are coming from. The light rays do not come together at the point where the image seems to be, so it cannot be captured on a screen. E.g. magnifying glasses. They are produced by diverging lenses.
What are the primary and secondary colours of light?
The three primary colours are red, blue and green.
The three secondary colours are magenta, cyan and yellow.
All of the primary colours together make white.
Why do objects look a certain colours?
Objects look a certain colour because they reflect that colour and absorb all the other colours.
Why does light refract?
Light refracts when it enters a medium that changes its speed. If it passes from a fast medium to a slow medium the light bends towards the normal. If it passes from a slow medium to a fast medium the light bends away from the normal.
What is dispersion?
When the light enters a prism the different colours of light travel at different speeds, so they diffract at different angles. When they leave the prism this effect is made greater. This splits the white light into different colours of light.
What is total internal reflection?
This is when all light is reflected back into the medium it is travelling through. This will happen if the angle of incidence is greater than the critical angle. This can only happen when the wave travels through a dense material like glass or water towards a less dense substance like air.
When a light ray enters hits a surface, what can it do?
If the angle of incidence is less than the critical angle, most of the light is refracted into the outer layer, but some if it is internally reflected.
If the angle of incidence is equal to the critical angle, the ray would go along the surface, with quite a bit of internal reflection as well.
If the angle of incidence is larger than the critical angle, no light comes out. It is all internally reflected - total internal reflection.
How do converging lenses work?
It is convex. It bulges outwards in the middle, causing parallel rays of light to be brought together at the principal focus (where rays hitting the lens parallel to the axis all meet).
How do diverging lenses work?
It is concave. It caves inwards, causing parallel rats of light to spread out. The principal focus is the point where ways hitting the lens parallel to the axis appear to all come from.
How is lens power calculated?
Lens power = 1 / focal length (m)
It is measured in dioptres (D)
It is positive for converging lenses and negative for diverging lenses.
What are the types of reflection?
Specular reflection - waves are reflected in a single direction by a smooth surface, so you get a clear reflection
Diffuse reflection - waves are reflected in all directions by a rough surface, so the surface is matt and you do not get a clear reflection
