# GCSE Physics P3 [Part 1]

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REFRACTION:

• Refractive Index (n) = SinI / SinR

Is caused by the waves changing speed when entering a new medium. The waves slow down and they bend towards the normal.

• Every transparent material has a refractive index. The refractive index of  a medium is the ratio of speed of light in a vacuum to speed of light in that medium.

Rules for refraction in a converging lens:

(1) An incident ray parallel to the axis refracts through the lens and passes through the principal focus on the other side.

(2) An incident ray passing through the principal focus refracts through the lens and travels parallel to the axis.

(3) An incident ray passing through the centre of the lens carries on in the same direction.

Rules for refraction in a diverging lens:

(1) 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).

(2) An incident ray passing through the lens towards the principal focus refracts through the lens and travels parallel to the axis.

(3) An incident ray passing through the centre of the lens carries on in the same direction.

LENSES:

Real image: Where light from an object comes together to form an image on a 'screen' - Like the image formed on an eye's retina

Virtual image: When the rays are diverging, so the light from the object appears to be coming from a completely different place. E.g.You can get a virtual image when looking at an object through a magnifying glass.

• A converging lens= Convex

• A diverging lens= Concave. Always produces a virtual image, image is upright, smaller than object and on same side of the lens as the object.

Axis= The line passing through the middle of the lens.

• To describe an image properly, you need to say 3 things:

(1) How big it is compared to the object

(2) Whether it is upright or inverted (upside down) relative to the object

(3) Whether its real or virtual

MAGNIFICATION:

Magnification= Image height / Object height

• The object being magnified must be closer to the lens than the focal length. Remember, "You can't project a virtual image onto a screen"

POWER:

• The more powerful the lens, the more strongly it converges rays of light, so the shorter the focal length. The power of a lens is given by this formula:

• Power= 1 / Focal length

• For a converging lens, the power is positive. For a diverging lens, the power is negative. The focal length of a lens is determined by two factor:

(1) The refractive index of the lens material

(2) The curvative of the two surface of the lens

• To make a more powerful lens from a certain material like glass, you just have to make it with more