Lecture 5 Flashcards
What is a prism dioptre?
Prism dioptre is used to name the system. A prism of 1 dioptre (∆) gives an apparent displacement of 1 cm to an object 1m away.
What is a centrad?
Centrad (inverted traingle) is sometimes used in place of dioptres. It is similar to prism dioptre’s but instead they use a arc of cirlce in place of a straight line.
What determines the strength of a prism usually?
The strength of a prism may be recorded according to the size of refracting angle. However, the total deviation is not specified in this format.
Explain what a +1 D and a - 1 D lens mean?
Parallel rays of light are brought to focus at a focal point. A lens with a focal distance of 1 m is said to have a refractive power of 1 D (dioptres). A stronger lens will have shorter focal lengths. A 2 D has a focal length of 0.5m focal length and a 5 D has a focal length of 1/5 m.
Rays of light can be converged or diverged. Convex lens of 1 D has a refractive power of + 1 D and a concave lens of 1 D has refractive power of - 1 D.
Use diagrams to explain how a cylinder may be thought of as a convex lens
A cylinder convex lens may be thought as a section of a solid cylinder
A concave cylindrical lens
A concave cylindrical lens can be formed by having a rectangular block with one surface indented by a solid cylinder
A spherical lens
- A spherical lens, similar diptric power notation is employed for cylindrical lens and obviously cylindrical lens only focus light in one plane and therefore it is important to record the axis of the cylinder
- There is a standard method in indicating the direction of the axis of cyindrical lens in Ophthalmic practice
- The standard specifies that the clinician is facing the patient, zero is at the clinician’s left and the scale is read below the horizontal with 90degrees at the bottom and 180degrees at the right side.
As for prisms, how can image formed by spherical lens be examined?
As for prisms, image formed by spherical lenses can be examined. Ifa a convex lens hed up before the eye and the lens is moved luttle from side to side, the image is seen to move in the opposite direction.
With the concave lens the image moves in the same direction.
What instruments are available to measure refractive power of lens?
Refractive power of lens can be measured by vertometer, lensometer, focimeter, refractionmeter, ultimeter…
Going from a low index to a high index medium the light will be refracted towards the normal
Conversely, going from high to low index, the light will be refracted away from the normal
When light travels from a dense to a less dense medium, the refracted light bend away from the normal. When the angle of incident light reaches critical angle the refracted angle equals to 90degrees.
If the angle of incidence light is larger than that of the critical angle, the incident light is reflected. As it occurs it obeys the laws of reflection.
Law of reflection
Law of reflection states that when light is reflected, the incident angle is equal to the angle of reflection.
Reflection
- When you look at the mirror the image seems to be located at a distance behind it
- Virtual images are located where the light rays can not actually reach, that is they can not be where the light rays from the image appear to be diverging from
- The image ft, the chimp is a real image where the image appears to have the light rays converging to its location
- Left to right reversal is related to the frame of reference of the viewer
Reflection by curved mirrors
First rule of reflection - concave mirror
A ray of light parallel to the principle axis will reflect through the focal point
Second rule of relfection - concave mirror
A ray of light passing through the focal point will reflect parallel to the principle axis
What will happen if an object is beyond the centre of curvature for concave mirrors
The image lies between N & F, is inverted, small and real
An object at the centre of curvature - concave mirrors
For an object between the centre of curvature and focal point - concave mirror
It produces a real image that lies beyond N, is inverted and larger than the object and is real
Object within the focal length - concave mirrors
The image is upright, larger than the object and virtual
Convex mirrors
- For convex mirrors, the centre of curvature and focal point are beyond the mirro - so the rules are different
- An incident ray parallel to the principle acis will reflect so that its extension will pass through the focal point
- An incident ray whose extension would pass through the focal point will be reflected parallel to the principle axis
Dispersion of light by a prism
Refractive index of a medium is to some extent depends on the frequency of light with the higher frequency of light is bent more. Due to the inversed relationship between wavelength and frequency, the short wavelength lights is refracted more than long wavelength of light.
Which is how we see rainbows.
