P3 - Refraction Flashcards
What is the formula that links the angle of incidence and angle of refraction?
Refractive index = sin i / sin r
Describe how and why refraction happens
When waves slow down, they bend towards the normal and when they speed up, they bend away from the normal (if a wave is already travelling in the direction of the normal, it slows down but doesn’t bend)
The incident ray hits the boundary at the angle of incidence, the refracted ray travels through the new medium at the angle of refraction and then the emergent ray exits the new medium at the angle of incidence
How do lenses work?
They form images by refracting light and changing its direction
What are the 2 types of lens?
Converging lens
Diverging lens
What is a converging lens?
A convex (bulges outwards) lens causes parallel rays of light to converge at the principal focus
What is a diverging lens?
A lens that is concave (caves inwards) and causes parallel light rays to diverge (spread out)
What are the 3 things you need to say to describe an image?
How big it is compared to the object
If it’s upright or inverted
If it’s real or virtual
What is the difference between a real image and a virtual image?
A real image is where the light from an object comes together to form an image on the display surface
A virtual image is where the rays are diverging so the light from the object appears to be coming from a completely different place
What is the axis of a lens?
A straight line passing through the middle of the lens
What is the principal focus?
For a converging lens, the principal focus is where rays hitting the lens parallel to the axis all meet
For a diverging lens, the principal focus is where the rays hitting the lens parallel to the axis appear to all come from
There is a principal focus on both sides of the lens
What is the focal length?
The distance between the centre of the lens and the principal focus
What are the 3 rules for refraction in a converging lens?
An incident ray parallel to the axis refracts through the lens and passes through the principal focus on the other side
An incident ray passing through the principal focus refracts through the lens and travels parallel to the axis
An incident ray passing through the centre of the lens carries on in the same direction
Describe how you draw a ray diagram for an image through a converging lens
Draw a horizontal line to be the axis and draw a vertical line through it with arrows on the ends pointing outwards to be the converging lens
Draw the object on the axis and draw the principal focus marked by F on the other side of the lens
Draw a ray from the top of the object parallel to axis until it hits the lens and refracts down to go through the focal point
Draw a ray from the top of the object through the centre of the lens and don’t refract it
Where the refracted rays meet is below the axis but is the top of an inverted image of the object. Draw the image of the object
Describe the 3 scenarios where the distance from the lens affects the image
An object at 2F (2 * principal focus) will produce a real, inverted image the same size as the object and at 2F
An object between F and 2F will make a image that is real, inverted and bigger than the object and beyond 2F
An object nearer than F will make a virtual, upright image that is bigger than the object and on the same side of the lens as the object (because the refracted light rays are still diverging meaning there will be an virtual convergence on the object side of the lens)
What are the 3 rules for refraction in a diverging lens?
An incident ray parallel to the axis refracts through the lens and travels in line with the principal focus so it appears to have come from the principal focus
An incident ray passing through the lens towards the principal focus refracts through the lens and travels parallel to the axis
An incident ray passing through the centre of the lens carries on in the same direction
Describe how to draw a diagram for an image through a diverging lens
The diverging lens is shown as a vertical line with arrowheads pointing inwards
Draw a ray from the top of the object through the centre of the lens
Draw a ray that is parallel to the axis from the top of the object to the lens and refract it so that it appeared to have come from the principal focus on the side of the lens of the object
Draw a dotted line from the principal focus on the object side of the lens to where the top ray refracts in order to show where the ray appears to be coming from
The top of the image is where the dotted line ray meets the other ray
How would you describe the image formed by a diverging lens?
The image is upright, smaller than the object, virtual and on the same side of the lens as the object
What type of lens is a magnifying lens?
Converging
How do magnifying glasses work?
The magnifying lens creating a magnified virtual image meaning that the object being magnified must be closer to the lens than the focal length
Why can’t magnifying lenses project images on to screens?
The light rays appear to be coming from a place that they are not coming from meaning the image is virtual
You can’t project a virtual image on to a screen
How do you work out magnification?
Magnification = image height / object height
What is very important when drawing ray diagrams?
To draw it very accurately because you might have to measure the heights of the object and image to work out magnification
How do you calculate the power of a lens?
Power = 1 / focal length
What 2 factors decrease the focal length?
Increasing the curvature of the surfaces of the lens
Increasing the refractive index of the lens
Why is making a lens more powerful useful?
You can increase the power of the lens to make allowances for reducing the lens thickness
How does changing the lens type (diverging / converging) affect the lens power?
Diverging lenses have negative power and converging lenses have positive power
What is the unit of lens power?
Dioptres
