Light Flashcards
Objects that emit light
Luminous objects
Objects that don’t emit light
Non-luminous objects
How can we see non-luminous objects?
From light that they reflect
Rule of reflection
A ray of light is reflected so that the angle of incidence is equal to the angle of reflection
Objects to change the direction of light and why
Mirrors eg periscopes Allows observers to see over or around objects
Plane mirrors
Images are created by objects in the room reflecting or emitting rays of lights which strike the mirror and are reflected into your eyes As your light rays normally ravel in straight lines, your brain interprets them as if they have come from in front of you and not from behind you (where they objects actually are)
Different images
Virtual images- Image in the mirror, rays of light do actually pass through them Real images- Rays of light are passing through them, eg a cinema screen
Properties of images in a plane mirror
- As far behind the mirror as the object is in front - Same size - Virtual - Laterally inverted
Speed of light
Vacuum/air= 300, 000, 000 m/s Water= 200, 000, 000 m/s
How does refraction happen/ what is it
When a ray of light travels through a boundary between two media eg air into water, it slows down This can cause the ray to change direction This change in direction is called refraction As the ray travels back to the original material it was travelling in, it returns to it’s original direction
When does refraction not happen
When the ray strikes the boundary at 90 degrees, it will not change direction
What is refractive index
The ratio between the sine of the angle of incidence and the since of the angle of refraction is constant This ratio is called the refractive index (n)
Refractive index equation
sin(i) n= ——- sin(r)
What is total internal reflection
Ray of light passes from an optically more dense medium to an optically less dense medium
Meaning of ‘optical density’
How much light slows down when it enters a material (different to regular density)
Setting up total internal reflection investigation
Detect a ray of lights at the centre of the straight side of a semi circular glass block through the curved side The incident ray always hits the edge of the glass at 90 degrees so there is no refraction of light entering the block
When is total internal reflection reached
If the angle of incidence in the glass is increased then all of the light is reflected from the boundary- this is called total internal reflection
When do the angle of incidence and the angle of reflection become critical angles
The angle of incidence is in the more dense medium This increases the angle of refraction until it becomes a critical angle This is when the refracted ray runs along the boundary of the two mediums
Critical angle in different mediums
Glass- 42 degrees Water- 49 degrees This is when the ‘less dense medium’ is air
Equation for the critical angle
1 sinC = — n
Using total internal reflection
Several partial reflections at the non silvered glass surface of a mirror create several faint images around the central main image in a plane mirror To avoid this, glass prisms can be used to alter the direction of light instead of mirrors
The prismatic periscope
Light enters at 90 degrees
Stikes diagonal surface at 45 degrees
Critical angle for glass is 42 so ray is totally internally reflected and turned through 90
When travelling through the second prism this happens again
Ray emerges parallel to the direction it was originally traveling in and image is brighter and hsarper
What is an optical fibre
Thin strand composed of two differnt types of glass
Central core is optically dense glass which has a high refractive index
This has a coat of optically less dense glass
How do optical fibres work
- Light enters the inner core and strikes the boundary of the two glasses at an angle greater than the critical angle
- No light escapes across this boundary
- The fibre works as a ‘light pipe’
- Large amounts of these fibres together is called a bundle which can carry sufficient light for images to be seen through them
- By tapering the image, it can also be maginfied
