Short Answer Questions

• 
  What type of lens is used to correct farsightedness?
• In farsightedness the eyes cannot see the near objects clearly. The lens of such eye is less convergent than the normal eye. The rays coming from a close object converges towards the point beyond the retina due to the less converging lens. A CONVEX LENS is placed in front of the eye to correct the fault.
• Which lens is of the high optical power: the one of short focal length or one of long focal length?
• The lens with short focal length is of higher optical power. As we know that the power of lens is defined as the reciprocal of the focal length where F is taken in meters i.e. D=1/f. It is clear from the above formula that power and focal length are inversely proportional to each other. Therefore increase in one causes a decrease in the other. Therefore shorter the focal length, higher would be the power  or viceversa.
• EXAMPLE: If focal length is 25cm then f=25cm=o.25cm. Power of lens is equal to 1/0.25cm+o.4m=40m.The power of convex lens is negative and that of concave lens is positive.

• Define Refractive index. How is it related to velocity of light?
• The ratio of sine of angle of incidence to the sine of angle of refraction is called refractive index. It is a constant and is denoted by “n”. This is also known as Snell’s law. Refractive index is equal to the ratio of speed of light in the two media. For example “c” is the speed of light in air and “v” is the speed of light in the media( glass). Then reflective index of glass will be: n=c/v. Since it is a ratio it has no unit.

• What is the condition of the angle of incidence for total internal reflection?
• As we know that the angle of incidence in the denser medium for which the corresponding angle of reflection in the rarer medium is 90 degree is called the critical angle. It is denoted by Ѳc. WHEN THE ANGLE OF INCIDENCE IN THE DENSER MEDIUM BECOMES GREATER THAN THE CRITICAL ANGLE Ѳc, there is no refracted beam. All the rays are internally reflected back in the optically denser medium. This phenomenon is called total internal reflection.