Chapter 5  (Spherical lenses)

1. The vergence power of a lens is dependent on:

a. its dispersive power

b. vergence power of each surface

c. thickness of the lens

d. the wavelength of the light

e. the medium on either side of the lens
 

Answera.F   b.T   c.T    d.F   e.T
The vergence power of a lens is affected by the vergence power 
of each surface, thickness of the lens and the medium on either side 
of the lens. The later explain why cornea is has a stronger refractive 
power than lens in human eye although its refractive index is less. 
. 
 
 

2. The following are true about the focal lengths of spherical lenses:

a. the first focal length always has the same length as the second focal length

b. the first focal length of a convex lens is to the right of the lens 

c. the first focal length of a concave lens is to the right of the lens

d. the second focal length of a convex lens has a positive sign 

e. the second focal length of a concave lens has a negative sign 
 

Answera.F  b.F  c.T  d.T  e.T
The first focal length has the same length as the second focal length only
if the medium on either side of the lens is the same. 
The first focal length of a convex lens is to the left of the lens whereas that of the concave lens is to the right of the lens.
By convention, the second focal length has a positive sign for convex lens and a negative sign for concave lens. As lenses are designated by their second focal lengths, a convex lens is also called a plus lens and a concave lens a negative lens. 
 
 

3. The following are true about magnification by the lens:

a. the linear magnification of an object is defined by the distance of the image 
    from the principal plane divided by the distance of the object from the 
    principal point.

b. angular magnification is more important than linear magnification in 
    determining the image size falling on the retina

c. a tall object situated far away may be perceived as having the same size as 
    a smaller one close to the eye

d. magnification of the depth of the image along the optical axis is known as 
    axial magnification

e. if the image is situated at infinity, its size is the same as its object size.
 

Answera.T   b.T   c.T   d.T   e.T
Image magnification can be defined in various ways:
Linear magnification is the quotient of the image height divided by 
the object height. 
Axial magnification is the magnification of the depth of the image along 
the optical axis.
Angular magnification is the quotient of the angle subtended by the image divided by the angle subtended by the object.
 
 

4. The magnifying glass:

a. increases the angle subtended by the image of a near object

b. has an angular magnification which is traditionally defined as angle 
    subtended by the image divided by the angle subtended by the same 
    object placed 25 cm from the eye

c. has an angular magnification equal to M/4 where M is the power of 
    the lens in dioptres 

d. is used to produce erect virtual image for the normal user

e. has a small field of vision 
 

Answera.T   b.T   c.T   d.T   e.T
The magnifying lens increases the angle subtended by the image of 
a near object placed in the primary focal plane of a plus lens.
For such a magnifier, the magnifying power is traditionally defined as
the quotient of the angle subtended by the image divided by the angle
subtended by the same object placed 25 cm from the eye. The magnifying power is calculated to be M/4 where M is the power of the lens in dioptres.

a
a
5. The increasing prismatic effect of the more peripheral parts of a spherical lens is
    responsible for:

a. ring scotoma

b. chromatic aberration

c. spherical aberration 

d. jack-in-the-box effect 

e. image distortion 
 

Answer
a.T   b.F  c.T  d.T  e.T
The increasing prismatic effect of the more peripheral parts of 
a spherical lens is responsible for:
spherical aberration
ring scotoma
jack-in-the-box effect
image distortion so that a thick plus lens gives a pin-cushion 
effect and a thick minus lens gives a barrel effect.