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Thermodynamics

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Visual Angle

It is the angle subtended by the object at the human eye. Consider the objects A₁ and A₂ placed in from of the eye. The objects are of different sizes, but they subtend equal angles on the eye and their images formed on the retina are of the same size. If k is the distance between the retina and the lens,

h1 ∝ θ1

Thus the size of the image on the retina is proportional to the angle θ. Suppose A₂ is brought nearer to the eye to a position A₃. When A₂ is at A₃, the size of the image on the retina h₂ and the visual angle is θ₂, such that

Consequently objects are brought nearer the eye to see then in more details but the object cannot be held very close to the eye because it cannot accommodate for very short distances. The difficulty is overcome by using telescopes and microscopes which project larger images on the retina than would be seen by unaided eye.

Angular magnification

Microscopes and telescopes are designed to increase the visual angle so that the object viewed can be made to appear large with their aid. Therefore the angular magnification M of the instrument is defined as the ratio of the visual angle subtended by the final image at the eye to the visual angle subtended by the object at the unaided eye.

Note: It us to be remembered that the visual angle is important and the size of the image are of less significance.