# AS Physics 2

Revision cards for Edexcel AS Physics unit 2

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## Refraction, Refractive Index & T.I.R

Refraction is when a wave changes direction as it enters a different medium. The change in direction is a result of the wave slowing down or speeding up.

Snell's Law uses angles to calculate refractive index (U). The refractive index of a material measures how much it slows down light. Snell's Law is written as -

1U2 = sin i/ sin r (going from medium 1 to medium 2)

The refractive index always has a value greater than 1.

Total Internal Reflection occurs when the angle of incidence is greater than the critical angle. The critical angle is the angle of incidence at which the angle of refraction is 90°. The wave is then also reflected, the inside surface of the material behaves like a mirror. TIR is when light goes from a slower, optically denser material to a less dense material (glass to air). Optical fibres use Total Internal Reflection.

U= 1/ sin C (C= critical angle)

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## The Doppler Effect

You can measure the speed of objects using the Doppler Effect.

When an object is moving away from you, the sound waves travel in the opposite direction from the object, so they are strectched out, have a longer wavelength and a lower frequency and pitch when they get to you.

The opposite happens when an object is moving towards you - the waves bunch up and have a shorter wavelegth and a higher frequency and pitch when they reach you.

How much the sound waves change depends on how fastthe object is travelling, the greater the objects speed, the larger the change. The Doppler Effect happens with all waves so it has a wide range of applications (e.g police radar guns using microwaves, speed of flowing blood using ultrasound sonography).

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## Light as a Photon

Light behaves like a wave or a stream of particles (photons). A photon is a quantum of EM radiation. Photons act as particles and will either transfer all or none of its energy when interacting with another particle, like an electron. Photon energy can be calculated by -

E=hf (where h is plancks constant - 6.63x10-34)

E= hc / wavelength (where C is the speed of light)

Photon energies are given in electrovolts (eV). An eV is the kinetic energy gained by an electron when it is accelerated through a potential difference of 1V. The equation for eV is the same as kinetic energy.1 eV = 1.6x10-19 J (e is the charge on an electron : 1.6x10-19 C) Photons are released from electrons in atoms. Electrons in atoms exist in energy levels. Electrons can move down an energy level by emitting a photon. The energy carried by each photon is equal to the difference in energies between the two levels. The equation below shows the transitiion between 2 levels.

change in E = E2 - E1 = hf = hc / wavelength

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## Longitudinal and Transverse Waves & Polarisation

A wave transfers energy away from its source. A progressive wave carries energy from one place to another without transferring any material. Waves can be reflected and refracted.

Reflection - wave bounced back after hitting aboundary Refraction - wave changes direction as it enters a different medium

Transverse waves are where the vibration is at right angles to the direction of travel. Longitudinal waves are where the vibrations are along the direction of travel. Sound is a longitudinal wave and can't go through a vacuum. Longitundinal waves rarefract and compress, the distance between them is the wavelength.

A polarised wave only oscillates in one direction. A polarising filter only transmits vibrations in one direction (e.g horizontal or vertical). If you have two polarising filters at right angles to each other no light will get through. Polarisation can only happen to transverse waves. When light reflects it is partially polarised. Television and radio signals are polarised. Some materials can rotate the plane of polarisation.

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## E.m.f, Internal Resistance & Potential Difference

Emf - (Electro-motive force) is the amount of electrical energy the battery produces for each coulomb of charge. Measured in volts (v).Potential Difference - (p.d) is the energy transferred when one coulomb of charge flows through the external resistor. Measured in volts (v).

Batteries have resistance, this comes from electrons colliding with atoms and losing energy. This is called Internal Resistance. The energy that is wasted overcoming the internal resistance is called the lost volts (v). If there was no internal resisitance the e.m.f would be the same as the p.d.

V = e - Ir (where V is the p.d., e is the e.m.f, r is the internal resistance)

e = I (R+r) (where r is the internal resistance, R is the external resistance)

e = V + v (where V is the p.d., v is the lost volts)

Most power supplies need a low internal resistance as they have to deliver a high current, e.g a personal stereo. Although, high voltage power supplies have very high internal resistances so if they're short circuited only a small current can flow, lots safer.

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Radiation Flux is power per unit area.

The amount of light falling on an area facing (at right angles to) the sun is called the radiation flux. E.g a small solar cell in strong sunlight could recieve the same amount of light as a larger cell in weak sunlight.

Radiation Flux (Wm-2) = Power (W) / Area (m2)

There are 2 ways to increase the amount of sunlight falling on a solar cell -

1. Increase the power of the light

2. Increase the surface area of the cell (physically or by angling the cell so it faces the sun more)

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