Light Flashcards
Angle of incidence, angel of reflection, normal
The angle of the wave approaching the boundary is called the angle of incidence (i)
The angle of the wave leaving the boundary is called the angle of reflection (r)
The line at right angles (90°) to the boundary is known as the normal
(Boundary being the platform, that ray hits and bounces off)
Reflection in a plane mirror
When an object is placed in front of a mirror, an image of that object can be seen in the mirror
The image will be:
- The same size as the object
- The same distance behind the mirror - as the object is in front of it
- Directly in line with the object
- virtual
- laterally inverted
This image u see in the mirror is called a virtual image
Refraction
Refraction is the bending of the light as it passes from one medium to another
At the boundary, the rays of light undergo a change in direction
The change in direction depends on which media the light rays pass between:
From less dense to more dense (e.g air to glass), light bends towards the normal
From more dense to less dense (e.g. glass to air), light bends away from the normal
The change in direction occurs due to the change in speed when travelling in different substances
When light passes into a denser substance the rays will slow down, hence they bend towards the normal
The only properties that change during refraction are speed and wavelength – the frequency of waves does not change
Refractive index
The refractive index is a number which is related to the speed of light in the material
The refractive index is a number that is always larger than 1 and is different for different materials
Objects which are more optically dense have a higher refractive index and the slower the light travels,
Objects which are less optically dense have a lower refractive index. The faster the light travels
Refractive index equation
N = sin i / sin r
(Less dense to denser medium)
N = refractive index of medium
Sin i =. Angle of incidence in vacuum
Sin r = angle of refraction
N= sin r / sin i
(More dense to less dense medium)
N = 1/sin C
C = critical angle
Refractive index = speed of light in a vacuum/speed of light in the medium
Remember the speed of light in air and a vacuum is 3 x 10^8
Total internal reflection
Sometimes, when light is moving from a denser medium towards a less dense one, instead of being refracted, all of the light is reflected
Conditions for total internal reflection
- The angle of incidence > the critical angle
- more dense to less dense medium
Examples
- Optical fibres e.g. endoscopes
- Prisms e.g. periscopes
Critical angle
As the angle of incidence is increased, the angle of refraction also increases until it gets closer to 90° (perpendicular to the normal)
Remember it has to be i < r
Optical fibres
Light travelling down an optical fibre is totally internally reflected each time it hits the edge of the fibre
Principal axis, principal focus/focal point, focal length
The principal axis
A line which passes through the centre of a lens
The principle focus
The point at which rays of light travelling parallel to the principal axis intersect the principal axis and converge
Focal length is defined as:
The distance between the centre of the lens and the principle focus
How to draw a ray diagram
- one straight line through the middle of the lens, start from the object and go through the middle of the lens, already diagonal
- draw a straight line from the top of the object to the top of the lens and then taking a sharp turn to go through the principal focus (f)
- when intersection of those two lines are the image
Converging (convex) lens
In a converging lens, parallel rays of light are brought to a focus
This point is called the principal focus
The distance from the lens to the principal focus is called the focal length
This depends on how curved the lens is
The more curved the lens, the shorter the focal length
Diverging lens
In a diverging lens, parallel rays of light are made to diverge (spread out) from a point
(Principal focus is backwards/on the other side the lens where the light is coming from)
Corrects short sightedness
Dispersion of light
White light is a mixture of all the colours of the spectrum
Each colour has a different wavelength (and frequency)
White light may be separated into all its colours by passing it through a prism
This is done by refraction
Violet light is refracted the most, whilst red light is refracted the least
This means that violet has the shortest wavelength and red has the longest wavelength
Reds wave speed is faster than violet speed
Going in rainbow order
This splits up the colours to form a spectrum
Remember when light refracted in a prism it produces a sad face
Beyond 2f, ray diagram
The image will be real inverted smaller and be between f and 2f
Camera
Between 2f and f, ray diagram
Beyond 2f real inverted larger
Projector
Object is on 2f, ray diagram
Real inverted same size image is at 2f
Between f and the lens, ray diagram
Image is behind object virtual erect and larger
Magnifying glass
