Optics Flashcards
What is light ?
An electromagnetic wave consisting of coupled electric and magnetic fields that vary periodically as they move through space.
The electric and magnetic fields are perpendicular and described as transverse in nature.
Examples of electromagnetic waves
Radio waves, X-rays and infra - red, and light
What is the distance between subsequent wave crests known as?
Wavelength. (λ)
What is the vertical height of the wave known as?
Amplitude.
What is the time taken for corresponding points on a wave to pass a fixed point known as?
Period (T)
What is the number of corresponding points passing in unit time known as?
Frequency (f).
What is the speed (c) of an electromagnetic wave in a vacuum?
3.0 x 10^8 m/s
How are speed, frequency and wavelength related in an equation?
c = f x λ
What does frequency (f) equal?
f = 1/T
T = time period
What is superposition of waves?
When two or more sinusoid waves act at the same place then the result can be found be adding the individual waves algebraically.
What is constructive interference?
If two waves have the same amplitude and frequency and are in phase, the waves will reinforce each other resulting in doubling their amplitudes.
What is destructive interference?
If two waves meet that are completely out of phase, at every point the amplitudes have the same magnitude but opposite sign, then they cancel each other out.
What is a ray?
Light treated as a form of energy which travels in straight lines.
What is a beam?
A collection of rays.
What is the law of reflection?
When a bean of light is reflected from a smooth surface, the angle of reflection equals the angle of incidence.
Øi = Ør
What is regular reflection?
If a parallel beam of light falls on a plane mirror, it is reflected as a parallel beam
What is diffuse reflection ?
If the surface is not smooth or is irregular, the surface will reflect the light diffusely (many directions).
What is lateral inversion?
How an object in a mirror appears to be further back within the mirror, and at the same distance as the object is in front of the mirror.
What can a laterally inverted image in a plane mirror be described as?
Unreal / virtual
What are the features of a concave mirror?
The centre of curvature (C) is in front of the mirror, the centre of the mirror is called pole (P).
A narrow beam of rays is reflected so that all rays coverage to a principal focus point (F) in front of the mirror and is described as REAL as the actually light passes through it.
What are the features of a convex (diverging) mirror?
The centre of curvature (C) is behind the mirror.
A narrow beam of rays is reflected to form a divergent beam which appears to come from a point behind the mirror.
It provides a virtual/unreal image.
What is spherical aberration?
Light rays from a point on a object that are reflected at different distances from the axis of spherical mirror do not converge at a single point.
How do rays behave under spherical aberration?
Rays reflected from outer parts of the mirror converge at focal points closer to the mirror than those reflected near the mirrors axis.
Therefore sharp images are produced at the axis of the mirror.
What do concave mirrors with parabolic cross sections produce?
High quality images that do not undergo spherical aberration.
Landing lights, search lights.
What three points should be considered when constructing ray diagrams?
- A ray travelling parallel to the principle axis, which after reflection actually passes through the principle focus or appears to diverge from it.
- A ray travelling through the centre of curvature which strikes the mirror normally is reflected back along the same path.
- A ray travelling through the principal focus is reflected parallel to the principal axis.
What image does ray diagram 1 produce ?
Real, inverted and diminished.
What image does ray diagram 2 produce?
Real, inverted and the same size as the object.
What image does ray diagram 3 produce ?
Real, inverted and magnified.
What is linear magnification?
A ratio between the size of image and the size of an object.
Magnification equation
Magnification = image height / object height
= image distance / object distance.
Laws of refraction
If light passes obliquely from air to water, its velocity changes which causes its direction to change. This is refraction.
The greater the ratio between the velocities in the mediums, the greater the refraction is.
The refracted ray is in the same place as the incident ray and the normal to the medium at the point of incidence but in the opposite side of the normal from the incident ray.
What happens if light goes from a medium of high velocity to a medium of low velocity? (Air into water)
It will refract towards the normal.
What happens if light goes from a medium of low velocity to a medium of high velocity? (Water into air)
It will refract away from the normal.
What is the index of refraction?
The ratio between the velocity of light in free space and it’s velocity in the medium.
n = c / V
Where
c = velocity of light in free space,
V = velocity of light in the medium
What is Snell’s law?
n1 x sin (i) = n2 x sin (Ø)
What is dispersion?
The bending power of the material dependant on the frequency.
Critical angle of incidence
SinØc = n2 / n1
What are the conditions for total internal reflection ?
The light ray must be attempting to travel from a medium of higher refractive index to a medium with a lower refractive index.
The angle of incidence must be greater than the critical angle Øc .
What is a lens?
A portion of transparent medium bounded by two curves.
Convex lenses are …
Thicker in the middle than at the edges and are known as converging lenses.
Concave lenses are …
Thinner in the middle than at the edges and are known as diverging lenses.
What is the principal axis of a lens ?
A straight line joining two centres of curvature and is perpendicular to the surfaces where it passes through them.
What is the principal focus of a lens?
The point in the principal axis to which all the rays close to and parallel to the axis, converge. Or, where they appear to diverge after refraction.
What is the optical centre of lens?
A fixed point for any particular lens and coincides with geometric centre of a symmetrical lens.
What is the focal length of a lens?
The distance from the principal focus to the optical centre of a lens.
What is a fibre optic?
A central core surrounded by a layer of material called cladding, followed by a jacket.
The core transmits the light waves and the cladding keeps the light waves within the core and provides strength to the core. The jacket provides protection from moisture and abrasion.
What are the advantages of fibre optics ?
They can carry signals with much less energy loss than copper cables and at a higher bandwidth, resulting in more information transmitted longer distances with less repeaters required.
They are thinner, lighter and require less insulation space.
