Counting Stars - Optical Instruments

What are the two things you mainly require to identify constellations, planets or just gaze at the stars?

A) A telescope

B) Your eyes

This blog covers the physics behind these two

Astronomical Telescopes - Refracting

They have a very simple job, to show far objects bigger and closer. To achieve this , they contain two lenses in them - The Objective Lens and the Eyeypiece , their focal lengths denoted by fo and fe respectively.

The Objective lens , as shown in the diagram, forms an intermediate image , which forms as an object for the eyepiece. The eyepiece forms a virtual - inverted and magnified image in the end.

Galilean Telescopes

These follow the same principles as a refracting astronomical telescope but the second lens differs here.

This can also be represented as :-

EYESIGHT

Our eyes itself is a portable lens-screen system attached to our head. The muscles present in them helps to stretch or push the lens, changing the focal length. This allows us to see through a wide range of distances.

But, at times, human eyes can develop various dieseases

1) Myopia - Nearsightedness

This means the person can't see far objects clearly. This is because the image by the eye lens is formed before the retina

This can be corrected by the use of concave lens

2) Hypermetropia - farsightedness

Yes, you guessed it, This means the person can't see objects near to them.

This is because the image formed by the lens is formed beyond the retina

This can be corrected by the usage of convex lens

3) Presbyopia

Presbyopia happens when the natural lens in the eye gets less flexible. This is a combination of both Myopia and Hypermetropia as the eye lens looses its ability to change its focal length. Its usually caused due to old age

One of the examples are:-

4) Astigmatism

Astigmatism is caused by a cornea or lens that has a different shape than normal. Normally, the cornea and lens at the front of the eye have an evenly round shape. This helps focus light rays sharply onto the retina so you can see clearly.

With astigmatism, light rays do not refractproperly as they enter the front of the eye. Vision is blurry at near and far because light rays either fall short of the retina or behind it.

One of the examples of Astigmatism :-

A meme to end this blog!