## Telescope and microscope

Telescope and microscope

### Astronomical Telescope

It is used for observing far off objects in the sky. In normal adjustment (final image at ∞), angular magnification/magnifying power, M= f0/-fe . When final image is screen at least distance of distinct vision (d), then M = f0/-fe (1- fe/d).

### Reflecting type telescope

In a reflection type telescope, objective lens is replaced by a large concave reflector of radius of curvature R M = f0/|fe| = (R/2)/ |fe|.

### Terrestrial telescope

It is used for observing far off objects on the ground. The essential requirement of such a telescope is that final image must be erect with respect to the object. To achieve it, an inverting convex lens is used in-between the objective and eye piece of astronomical telescope. This, however, increases the length of telescope tube. In Galilean telescope, no additional lens is used. Instead, the eye lens is a concave lens, instead of being convex. This, of course, reduces the field of view of Galilean telescope.

### Power of a microscope

Resolving power of a microscope is given by R.P. = 1/d=2 μsin θ/λ Where d is minimum distance between two point objects which can just be resolved, λ is wavelength of light used, μ is refractive index of medium between object and objective lens. The distance (d) is a measure of limit of resolution of the microscope. Clearly, smaller is the limit of resolution; greater is the resolving power of the microscope.

### Power of a telescope

Resolving power of a telescope is given by R.P. = 1/dθ =D/1.22λ Where dθ is angular separation of two stars which are just resolved in the telescope, D is diameter of objective lens and λ is wavelength of light used. Note that dθ is a measure of limit of resolution of the telescope. Smaller the limit of resolution, greater is the resolving power of telescope.

### Simple microscope or magnifying glass

It is used for observing magnified image of tinny objects .It consists of a single convex lens of small focal length. The angular magnification or magnifying power, M= (1+d/f), where d is least distance of distinct vision and f is focal length of the lens.