Refraction

KEYWORDS: refraction, angle of incidence,Angle of refraction, refractive index

Understand the phenomena of

refractionALL – Describe what happens during refractionMOST – Explain why refraction occursSOME – State the relationship between the angle of incidence and the angle of refraction

StarterRefraction mind

map

Refraction

Refraction occurs when light moves between two mediums that have a different density. (The word medium just means ‘something that you can travel through’)

When light moves into a medium with a higher density, it slows down and will bend towards the normal. When it moves into a less dense medium, it speeds up and bends away from the normal.

LO: Understand the phenomena of refraction

Refractive Index

The refractive index of a material is a measure of how much light is refracted by it when it passes through the material. It can be calculated by using the equation:

Refractive index = Sin I / Sin R

LO: Understand the phenomena of refraction

I = angle of incidenceR = angle of refraction

Demonstration: Critical Angle

When performing refraction with a semi-circular block, an interesting phenomena is observed. When the light leaves the block, it bends away from the normal. This is expected as it is moving from a more dense to a less dense medium.

LO: Understand the phenomena of refraction

Demonstration: Critical Angle

As we continue to increase the angle of incidence, we reach a point where the refracted light seems to run across the top of the glass block! The angle at which this happens is called the ‘critical angle’

LO: Understand the phenomena of refraction

Demonstration: Critical Angle

If we increase the angle even more, we can see that the light ray is no longer refracted. Instead, it seems to reflect off inside of the straight edge of the semi-circular block. This is known as total internal reflection

We will explore this in greater detail later

LO: Understand the phenomena of refraction

Critical Angle

The critical angle of a material can be found using the following formula:

Refractive index = 1/sin(C)

LO: Understand the phenomena of refraction

Refractive index = property of a material C = Critical angle

Uses of Total Internal Reflection

Fibre optic cables are able to work using total internal

reflection. Light is sent through one end in short bursts and will

light up the other end.

These fibre optic cables are now used in high speed internet! They are now also used in

medicine…

LO: Understand how physics can be applied to medicine

Uses of Total Internal Reflection

Total internal reflection is put to good use in Endoscopes. These are cameras that can be used to

look inside patients without having to perform invasive

surgery.

Light is shone into one end, travels through the endoscope,

shines off the inside of a patient, back along the endoscope and

forms a picture!

LO: Understand how physics can be applied to medicine

Lenses

KEYWORDS: converging, diverging, lenses, Focal point, virtual, real

Understand how to draw ray diagrams for

lenses

ALL – state the definition of converging and divergingMOST – Draw ray diagrams for lensesSOME – Explain when virtual and real images are formed when using lenses

StarterMake a mind map of all the objects that

you can think of that use lenses!

Converging lensLO: understand how to draw ray diagrams for lenses

A converging lens is always convex. It makes rays that are coming in that are parallel converge onto a point. The point where the rays converge is called the principal focus, or focal point.Converging lenses are used in magnifying glasses and in cameras.

Focal lengthLO: understand how to draw ray diagrams for lenses

For both lenses, the distance between the centre of the lens and the focal point is called the focal length.

Real imageLO: understand how to draw ray diagrams for lenses

When an object is really far away from a converging lens, the light rays will be (almost) parallel when they reach the lens. The image that will be formed will be at the focal length.

We call this image a real image.

Real imageLO: understand how to draw ray diagrams for lenses

A real image is an image that is formed by a converging lens if the object is further away than the principal focus.

The real image will always be smaller than the actual object, inverted and forms after the lens.

Virtual imageLO: understand how to draw ray diagrams for lenses

A virtual image is formed by a converging lens if the object is nearer to the lens than the focal length. A diverging lens will ALWAYS form a virtual image. The image is always bigger than the object, the right way up and forms before the lens.

MagnificationLO: understand how to draw ray diagrams for lenses

The magnification produced by a lens can be worked out using the following formula

Magnification = image heightobject height

Example 1LO: understand how to draw ray diagrams for lenses

The image produced by a magnifying glass is 10cm. If the height of the object is 2cm, calculate the magnification produced by the lens

Example 2LO: understand how to draw ray diagrams for lenses

A lens has a magnification factor of 4. How large will a 7cm object viewed through the lens be?

Power of a lens

The power of a lens can be worked out using the following equation:

Power = 1

LO: understand the structure of the eye

Focal length

Power = Dioptre (D)Focal length = m