Electromagnetic Waves

All the stars in the universe emit radiation in the form of electromagnetic waves which are oscillating electric and magnetic fields perpendicular to each other in the direction of propagation.

The distance between two consecutive crests is called wavelength denoted by λ. The time interval between two crests is called the time period denoted by T. The inverse of the time period is called frequency denoted by v. frequency (v)=1/T. The units of frequency are hertz. The speed of light in terms of frequency and wavelength is denoted by c=vλ.

The light travels fastest in a vacuum at 3 X 10^8 m/s.

In quantum mechanics, the electromagnetic wave is defined as a beam of particles called photons moving at a speed c in a vacuum. Its energy is defined as E=hv=hc/λ

where Planck's constant h=6.626 X 10^-34 kg/m^2

Based on the light emitted by different atoms or ions present at the surface of stars we can calculate the surface temperature, density and composition of different elements.

Electromagnetic spectrum:

We observe the universe at different wavelengths and frequencies such as Gamma, Ultraviolet,x-rays, visible, Infrared, microwaves and radio. visible light is the most common among them.

Not all wavelengths are used for ground-based observations because of atmospheric absorption. The earth's atmosphere act as a shield that prevents gamma, ultraviolet and x-rays from reaching the earth. The microwaves are also absorbed by the atmosphere. So only the visible and Radio waves having 10m wavelengths are used for ground observations as radio waves larger than 10m are terminated by the ionosphere. The visible light ranges from 390 nm to 780 nm.

Refraction: When light passes from one medium to another medium it changes its direction due to a change in speed.

Snell's law: n₁ sin θ₁ = n₂ sin θ₂ where n₁,n₂ are refractive indexes and θ₁,θ₂ are angle of incidence and refraction

Diffraction: The bending of light rays around an obstacle or corner is called diffraction.

Telescopes

A telescope consists of two main parts objective and eyepiece. The objective receives the light from distant astronomical objects and forms an image while the eyepiece is the ocular lens through which we see the image.

The two main characteristics of telescopes are

Light Gathering Power(LGP): It measures the amount of brightness of the image

Resolving Power: It's the ability that a telescope can distinguish between two objects in an image that may be close to each other.

In general, we see objects in space at very far distances from earth whose angular separation θ is very small. The minimum angle at which we can differentiate two objects is called the resolution of the telescope. When angular separation θ less than the minimum angle then two objects get overlap.

Types of Telescopes

1. Refractor Telescope: In refractor telescopes convex lens is used as an objective. The parallel rays of light get refracted twice because of two convex lenses and focussed at a point known as a focal point and the distance from the center of the lens is called the focal length(f).

   The beam of light from the objective lens which has a larger area than the eyepiece is focused onto a smaller area such that the brightness of the image is increased. Increasing brightness is one of the important properties of a telescope. Light-gathering Power is directly proportional to the square of the radius of the objective lens. LGP ∝ R^2.

   

   Telescope also produces a magnified image of astronomical objects. let A & B are two objects that subtend an angle θi on the telescope lens and θe be the angle subtended at the eyepiece then the magnification is given by m = θe/θi. In terms of focal lengths, the magnification is given by m = fo/fe where fo, fe are focal lengths of the objective and eyepiece.

   The primary disadvantage of the Refractor telescope is chromatic aberration. Where the visible light coming from distant objects with different frequencies appears at different positions which makes the image looks distorted due to refraction.

2. Reflector Telescope: Reflector telescopes use a concave lens as an objective which is also known as a primary mirror where the parallel beam of light is focussed to a point known as a focal point to record the image using a camera or digital imaging methods.

   The image formed by the objective can be viewed by many methods like prime focus where the camera or digital imaging device can be placed at the focal point. In Newtonian focus, a secondary mirror is used to direct the light out of the telescope through an opening on one side of the telescope. In Cassegrain focus, a secondary mirror is used to direct the light out of the telescope through a hole in the objective lens.

   

   Modern telescopes including Hubble and James Webb Telescopes are reflector telescopes. Reflector telescopes have many advantages over refractor telescopes and the primary advantage is that it does not suffer from chromatic aberration.

The ground based telescopes suffer from atmospheric fluctuations due to refraction of the atmosphere where the light coming from source object will be scattered in different directions and frequencies which can lead to reduced intensity of light. Telscopes can be mouted in two ways.

Equatorial Mount: In this mount we have two axis one is polar axis and other is equatorial axis. we should move the telescope around polar axis to track the object due to rotation of earth.

Azimuthal Mount: In this mount one of the axis point to local vertical and other points to horizontal. In this mount we have to move both the axis to track the object wrt to earth rotation.

Apart from visible light we can also observe the universe at different wavelengths such as x-ray, radio waves and Infrared .