# OCR Gateway B Module P5: Space for Reflection

## Gravity:

• All objects attract other objects - gravity is a universal force of attraction between masses
• A satellite is an object which orbits a larger object in space, because of gravitational force towards the centre of the larger object, centripetal force. Motion is at a tangent
• Gravitational force is directly proportional to `1 ÷ distance2`
• Comets travel faster when close to the sun because of a stronger gravitational force

## Polar orbits:

• Satellites in a polar orbit will orbit above the poles, generally between 100 km and 400 km above the surface, taking under 90 minutes per orbit (depending on altitude)
• The International Space Station is in a polar orbit, and this orbit is also used for satellite imaging and short range weather forecasting

## Geostationary orbits:

• These orbit the Earth around 36000 km above the Earth's surface and each orbit takes exactly 24 hours. They orbit around the equator and therefore remain in a fixed position above the Earth's surface

## Orbit period and distance:

• `Distance travelled by satellite = 2πr, where r is the distance between the centre of the Earth and the satellite`
• `Time of orbit = orbit distance ÷ speed`

## Equations of motion ('SUVAT equations'):

• You don't have to memorise these, they are at the front of the question paper
• What the letters represent:
- u = initial velocity (m/s)
- v = final velocity (m/s)
- a = acceleration (m/s2)
- s = distance travelled (m)
- t = time taken (s)
• `v = u + at`
• `s = 0.5(u + v)t`
• `s = ut + 0.5at2`
• `v2 = u2 + 2as`

## Vectors and scalars:

• A scalar quantity has a magnitude only (e.g. speed)
• A vector quantity has a magnitude and direction (e.g. velocity)

## Projectile motion:

• The path of a projectile is called the trajectory and is curved
• The optimal angle for the greatest distance is 45° (↗)
• Newton said that a ball kicked or shot from a tall peak at a high enough speed would orbit the Earth
• Ignoring air resistance, for a projectile in Earth's gravitational field,
- There is no horizontal acceleration (horizontal velocity is constant)
- If launched horizontally, the projectile will fall as the vertical acceleration increases steadily due to gravity

## Force pairs:

• There is always an action and a reaction - both are equal in magnitude and opposite in direction; this is Newton's third law
• For example, in a rocket, the force pushing the particles downwards equals the force pushing the rocket upwards
• In a collision of two objects going in the same direction, `the momentum after the collision = the momentum of object one before the collision + the momentum of object two before the collision` (`m1u1 + m2u2 = (m1 + m2)v`)
• Remember, `momentum = mass x velocity`

## Kinetic theory of gases:

• Moving particles in materials have kinetic energy
• They have more energy at high temperatures
• In gases, particles are free to move
• When colliding with walls, a force (pressure) is created on the walls:
- Particles hit the walls more often if the room is smaller - higher pressure
- Particles hit walls more often if the temperature is higher

## Satellite communication:

• Parabolic transmitters are used to send microwaves into space, parabolic receivers are used to receive them from the geostationary satellites
• Digital signals are used because there is less noise and they don't attenuate (lose energy) as quickly
• The ionosphere reflects radio waves under around 30 MHz frequency
• Waves over 30 GHz are absorbed and scattered by the ionosphere
• Therefore, 3 - 30 GHz is generally used for satellite communications, lower frequencies for low orbit satellites and higher ones for geostationary
• Geostationary satellites (used for communications) are all in the same orbit, which is very crowded. The receiving and transmitter dishes need alignment and need to be larger than the wavelength of the wave. Microwaves don't spread out much due to their relatively small (1mm - 300mm) wavelength
• TV aerials must be in line of sight with the transmitter
• Radio waves have long wavelengths compared to distances between hills so they diffract easily

## Interference:

• Coherent light consists of waves with the same frequency, amplitude and phase, e.g. lasers
• Laser light is monochromatic - all waves have the same frequency and wavelength
• When multiple waves overlap there is interference:
- When two crests or two troughs overlap (when the waves are in phase), there is constructive interference - the wave is amplified
- When a trough and crest overlap, there is destructive interference
• When light passes through a single slit it diffracts
• When it passes through two slits which are close together, there will be interference and dark/light bands will be produced

## Light as a wave:

• Light travels in straight lines - shadows and eclipses support this theory
• It has not always been thought to be like this, though. Newton thought of light as a particle. If he was correct, light should travel faster in a denser medium. Huygens thought of light as a wave - travelling slower in denser mediums. Newton was proved wrong when the speed of light was finally measured
• However, light can 'bend' under certain circumstances
• All electromagnetic waves are transverse
• Interference between light waves is produced when light diffracts as it passes through narrow slits
- Bright bands are constructive interference
- Dark bands are destructive interference
• Light has many planes of oscillation, it's unpolarised (shown in the diagram, from a perspective looking straight towards the light source) Light is polarised if all oscillations are only in one direction at right angles to the wave direction
• Polaroid sunglasses only allow oscillations in one direction through, reducing the amount of light passing through

## Refraction:

• More details and diagrams on reflection and refraction can be found in the P1 module
• Refraction happens when light enters a different medium and the speed therefore changes
• Light slows as it enters a more dense medium, with the angle of refraction smaller than the angle of incidence (it deviates towards the normal)
• Light speeds up as it enters a less dense medium, with the angle of refraction greater than the angle of incidence (it deviates away from the normal)
• The refractive index shows the amount of deviation - it's higher if there is lots of deviation:
`refractive index = speed of light in vacuum ÷ speed of light in medium`
• The speed of light in a vacuum is 300,000 km/s and the speed of light in glass is 200,000 km/s. So the refractive index is `300000 ÷ 200000 = 1.5` in glass, so it bends more than in air (1.0 refractive index), and in water (1.33 refractive index)

## Dispersion:

• Dispersion happens because each colour slows down by a different amount when entering a medium and speeds up by different amounts when leaving the medium
• Therefore, dispersion always happens when white light is refracted
• The spectral colours are as follows:
- red (highest wavelength)
- orange
- yellow
- green
- blue
- indigo
- violet (lowest wavelength)
These are often remembered as ROYGBIV or "Richard of York Gave Battle In Vain"
• Blue light has a greater refractive index than red light

## Critical angle:

• The critical angle is the angle of incidence in the more dense medium that produces an angle of refraction of 90° in the less dense medium, shown here: • If the angle of incidence is greater than this, then total internal reflection (TIR) occurs. TIR can be used in optical fibres, binoculars and reflectors/cat's eyes on the road and on road signs

## Magnification:

• `magnification = image size ÷ object size`

## Convex lenses:

• Convex lenses are thicker in the middle than at the edges
• They are also called converging lenses
• They can be used in cameras, projectors, some spectacles and as a magnifying glass

## Focal points and images:

• The principle axis is at 90° to the face of the lens and passes through its centre
• Light rays coming in parallel to the principle axis are refracted to a focal point on the other side of the lens on the principle axis. If the lens is "fatter", the focal point is closer to the lens
• Light rays not parallel to the principle axis are focused at the other side of the principle axis
• When an object is closer to the lens than the focal point, a virtual image is produced, like in a magnifying glass. Virtual images cannot be projected onto a screen or film because the image is formed on the same side of the lens than the object