Physics Revision: Waves

General properties of waves

Waves transfer energy and information without transferring matter

In a transverse wave, the oscillations are PERPENDICULAR to the direction of energy transfer.

In a longitudinal wave, the oscillations are PARALLEL to the direction of energy transfer. Longitudinal waves show areas of compression and rarefaction.

Electromagnetic waves are transverse, sound waves are longitudinal, and mechanical waves can be either.

Waves can be reflected, refracted and diffracted.

When identical sets of waves overlap, they interfere with each other

WAVE SPEED= FREQUENCY X WAVELENGTH

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The Electromagnetic Spectrum

In order of increasing frequency and decreasing wavelength:

• Microwaves= Mobile phones and satelitte television
• Infrared= Remote controls
• Visible light=Photography
• Ultraviolet= security marking
• X-rays=medical imaging
• Gamma rays=Sterlising medical instruments and killing harmful bacteria in food.

Exposure to these can be hazardous:

• Microwaves=Heating of body tissue
• Infrared=Skin burns
• Ultraviolet=Skin cancer and blindness
• X-rays=High doses kill cells (Transmitted by soft tissue, Absorbed by metal and bone)
• Gamma rays=Genetic mutations
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Sound and Ultrasound

Sound waves are longitudinal waves, and causes vibrations in a medium known as sound.

The range of human hearing is from 20Hz to 20000Hz

The pitch of a sound depends on frequency and loudness by its amplitude (height of the wave)

Sound waves can be reflected (echoes) and diffracted

Ultrasound is acoustic energy in the form of waves with a frequency above the range of human hearing.

Ultrasound waves partially reflect when they meet a boundary between two different media. The time taken for reflections to reach a detector can determine how far away the boundary is.

DISTANCE BETWEEN INTERFACES= WAVE SPEED X TIME TAKEN

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Reflection

When waves are reflected:

The angle of incidence=The angle of reflection

The normal is a line perpendicular to the reflecting surface at the point of incidence.

The image produced in a plane mirror is virtual

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Refraction and total internal reflection

Refraction is the change in direction of light waves when they pass from one medium to another:

• When light enters a more dense medium it refracts towards the normal
• When light enters a less dense medium it refracts away from the normal
• Waves are not refracted if travelling along the normal
• Waves are refracted due to change of speed.

Refraction in a prism can lead to dispersion.

The refractive index(of a medium)= SPEED OF LIGHT IN VACUUM

SPEED OF LIGHT IN THE MEDIUM

REFRACTIVE INDEX=sin(angle of incidence)/sin(angle of refraction)

OR 1/sin(critical angle)

Total internal reflection occurs if the angle of incidence within the more dense medium is greater than the critical angle. This is used in optical fibres to transmit visible light (endoscope)

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Lenses and the eye Part 1

In a convex (or converging) lens, parallel rays of light are brought to a focus at the principal focus.

The distance from the lens to the principal focus=The focal length

The focal length is determined by:

• The refractive index of the material from which the lens is made
• The curvature of the two surfaces of the lens.

In a concave (or diverging) lens, the rays of light diverge as if coming from the principal focus.

1/FOCAL LENGTH=1/OBJECT DISTANCE + 1/IMAGE DISTANCE

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Lenses and the eye Part 2

MAGNIFICATION=IMAGE HEIGHT/OBJECT HEIGHT

POWER OF A LENS= 1/FOCAL LENGTH

The eye contains the following:

• Retina
• Lens
• Cornea
• Pupil/Iris
• Ciliary Muscle
• Suspensory ligaments

The eye can focus on objects between the near point (usually 25cm) and the far point. This is called the range of vision.

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Lenses and the eye Part 3

Lenses can correct defects of vision:

• Long sight, when the eyeball is too short or the lens can't focus
• Short sight, when the eyeball's too long or the lens can''t focus

The structure of the eye is similar to that of a camera, as the eye brings an image to focus on the retina by changing the shape of the lens, whilst a camera focuses an image by varying the distance between the film and the lens. THE FILM=THE RETINA

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Red-shift

If a wave source is moving, relative to an observer, then there will be a change in the observed wavelength and frequency. This is THE DOPPLER EFFECT.

As the source moves away from the observer, the observed wavelength increases and the frequency decreases, and viceversa for when the source moves closer to the observer.

There is an observed increase in the wavelength of light from distant galaxies.The further away the galaxy, the faster they're moving and the larger the observed increase in wavelength. This is called red-shift.

This is evidence the universe is expanding and supports the Big Bang.

Cosmic Microwave Background Radiation (CMBR) is a form of radiation filling the universe, and can only be explained by the the Big Bang theory

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