THERMAL RADIATION

   THERMAL RADIATION 

The radiation emitted by a body as a result of its temperature is called thermal radiation.

All bodies emit such radiation to their surroundings and absorb such radiation from them. If a body is at first hotter than its surroundings, it will cool off because its rate of emitting energy exceeds its rate of absorbing energy. When thermal euilibxium_is reached the rates of emission and absorption are equal.Matter in a condensed state (i.e., solid or liquid) emits a continuous spectrum of radiation. The details of the spectrum are almost independent of the particular material of which a body is composed, but they depend strongly on the temperature. At ordinary temperatures most bodies are visible to us not by their emitted light but by the light they reflect. If no light shines on them we cannot see them. At very high temperatures, however, bodies are self-luminous. We can see them glow in a darkened room; but even at temperatures as high as several thousand degrees Kelvin well over 90% of the emitted thermal radiation is invisible to us, being in the infrared part of the electromagnetic spectrum.Therefore, self-luminous bodies are quite hot.

Periodic Motion

Periodic Motion

   If a particle moves such that it retraces its path regularly after equal intervals of time, its motion is said to be periodic. This interval of time, required to complete one round cycle of motion, is called period.

Newton's First law

Newton's First law

   According to this law, an object at rest tend tend to stay at rest and an object in motion tend to stay in motion, with same speed in the same direction unless it is compelled by same external unbalanced force to change that satat. This law provides the qualitative definition of force.

Example of Newton's First law

   The card and coin experiment 

   Take a smooth card and keep it on a glass. Place a coin on the card and flick sharply in the horizontal direction. The cord flies away and the coin drops into the glass.

    Initially, there are tow forces on the coin. The earth pulls the coin downloads and the card pushes it upwards. The forces balance each other and the coin remains at rest.

    When we apply a horizontal force on the card,  it is accelerated and it moves away. Since the friction between the card and the coin is negligible there is no force on the coin in the horizontal direction. It remains in its original position due to inertia of rest. 

   When the card moves away from underneath the coin, the normal force on the weight is the only force on the coin becomes zero. The weight is the only force on the coin and hence, it accelerates downwards to fall into the glass.

Refraction of light when it goes from a Rarer Medium to a Denser Medium

Refraction of Light When it Goes From a Rarer Medium to a Denser Medium

 From figure  shows a ray of light AO going from air (a rarer medium) into glass (which is a denser medium). In this case, the  incident ray AO gets refracted at point O and bends towards the normal ON' and goes in the direction OB inside the glass slab. Thus, when a ray of light goes from air into glass, it bend towards the normal (at the point of incidence). In this case, the angle of refraction (r) is smaller than the angle of incidence (i). Water is also an optically denser medium than air, so when a ray of light goes from air into water, it bends towards the normal. Thus, the refraction of light on going from air to water is similar to the refraction of light from air to glass (which has been shown in Figure ). To show the path of a ray of light going from air into water, we can use Figure 4 but we will have to write ‘water’ in place of ‘glass’ and in place of glass slab we will have to show water by drawing some dotted lines. Please draw the diagram to show the refraction of a ray of light from air to water yourself. 

Refraction of Light

Refraction of light

   Light rays travel in straight lines in a homogeneous medium. But whenever a light ray passes from one transparent medium to another, it deviates from its original path at the interface of the two media. In the second medium the ray either bends towards the normal to the interface, or away from the normal. The bending of the light ray from its path in passing from one medium to the other medium is called 'Refraction' of light.

    Let a plane surface separating two transparent media, Medium 1 and Medium 2. Let their refractive indices be n₁ and n₂ respectively. The two media can be any pair such as air and water, or air and glass. Suppose incident ray traveling in medium 1 and  refracted ray traveling in medium 2 shown in figure. Draw a normal . The angle i between the incident ray and the normal is called the angle of incidence. The angle r between the refracted ray and normal is called the angle of refraction.

    The refraction of light takes place according to the following two laws known as  the  laws of refraction :

1. The incidence ray, the refracted ray and the normal to the interface at the incident point all lie in the same plane.

2. The angle of incidence and the angle of refraction satisfy the equation.

                                                   sin i/sin r

This law is called Snell's law and the constant the refractive index of the second medium  with respect to first medium.