Quick Review Flashcards - Click to flip and test your knowledge!
Question
What is a lens?
Answer
A transparent refracting medium bounded by either two spherical surfaces, or one spherical surface and the other a plane surface.
Question
A plane surface in a lens can be treated as a spherical surface with an infinite _____.
Answer
Radius of curvature
Question
What are the two general kinds of lenses based on their effect on light rays?
Answer
Convex (converging) lens and concave (diverging) lens.
Question
Term: Convex lens
Answer
Definition: A lens that is thick in its middle and thin at the periphery, causing light beams to converge.
Question
Term: Concave lens
Answer
Definition: A lens that is thin in its middle and thick at its periphery, causing light beams to diverge.
Question
How is a concavo-convex lens physically distinguished from a convexo-concave lens?
Answer
A concavo-convex lens is thicker in the middle, whereas a convexo-concave lens is thinner in the middle.
Question
Why is a convex lens considered to be made of a set of prisms with bases directed towards the centre?
Answer
Because light incident on the upper and lower parts bends towards the base of each prism, causing the rays to converge.
Question
Concept: Action of a lens as a set of prisms
Answer
The central part of a lens acts as a parallel-sided glass slab, passing light rays normally incident on it undeviated.
Question
What is the 'centre of curvature' of a lens surface?
Answer
The centre of the sphere of which the lens surface is a part.
Question
The line joining the centres of curvature of the two surfaces of a lens is called the _____.
Answer
Principal axis
Question
What is the 'optical centre' of a thin lens?
Answer
A point on the principal axis such that a ray of light directed towards it passes undeviated through the lens.
Question
For a convex lens, the first focal point ($F_1$) is a point on the principal axis such that rays originating from it become _____ to the principal axis after refraction.
Answer
Parallel
Question
For a concave lens, how is the first focal point ($F_1$) defined?
Answer
The point on the principal axis towards which incident rays appear to meet before refracting parallel to the principal axis.
Question
Where do rays incident parallel to the principal axis of a convex lens meet after refraction?
Answer
At the second focal point ($F_2$).
Question
What is a 'focal plane'?
Answer
A plane normal to the principal axis passing through the focus.
Question
How is 'focal length' ($f$) of a lens measured?
Answer
The distance of the focus from the optical centre of the lens.
Question
Under what condition are the first and second focal lengths of a lens equal?
Answer
When the medium is the same on both sides of the lens.
Question
How does the refractive index ($\mu$) of a lens material relative to its surroundings affect its focal length?
Answer
If the refractive index decreases (e.g., lens placed in water), the focal length increases.
Question
How does lens thickness affect focal length if the material is the same?
Answer
A thick lens has a shorter focal length than a thin lens.
Question
If a part of a lens is covered, what change occurs in the image produced?
Answer
The intensity of the image decreases, but its position, size, and nature remain unchanged.
Question
Contrast: Real image vs Virtual image
Answer
A real image is formed by the actual intersection of refracted rays and can be caught on a screen, while a virtual image is formed by rays that only appear to meet and cannot be caught on a screen.
Question
For a convex lens, what are the characteristics of the image when the object is at infinity?
Answer
Real, inverted, and highly diminished (point-sized) at the focus $F_2$.
Question
At what position of the object does a convex lens produce a real, inverted image of the same size as the object?
Answer
When the object is at $2F_1$.
Question
Describe the image formed by a convex lens when the object is placed between the focus $F_1$ and the optical centre $O$.
Answer
Virtual, erect, and magnified.
Question
What application uses a convex lens to produce an erect and magnified image?
Answer
A magnifying glass (or simple microscope).
Question
What are the three universal characteristics of an image formed by a concave lens for any finite distance of the object?
Answer
Virtual, erect, and diminished.
Question
Where is the image formed by a concave lens if the object is at infinity?
Answer
At the focus $F_2$, on the same side as the object.
Question
State the lens formula relating $u$, $v$, and $f$.
Answer
$\frac{1}{v} - \frac{1}{u} = \frac{1}{f}$
Question
According to the sign convention, what is the sign of the focal length for a convex lens?
Answer
Positive
Question
According to the sign convention, what is the sign of the focal length for a concave lens?
Answer
Negative
Question
In lens sign conventions, distances measured in the direction of the incident ray are taken as _____.
Answer
Positive
Question
Concept: Linear Magnification ($m$)
Answer
Definition: The ratio of the height of the image ($I$) to the height of the object ($O$).
Question
What is the formula for linear magnification ($m$) in terms of image distance ($v$) and object distance ($u$)?
Answer
$m = \frac{v}{u}$
Question
What does a negative value for linear magnification ($m$) imply about the image?
Answer
The image is real and inverted.
Question
What does a positive value for linear magnification ($m$) imply about the image?
Answer
The image is virtual and erect.
Question
Concept: Power of a lens
Answer
Definition: The measure of the deviation produced by a lens on light rays incident on it, expressed as the reciprocal of its focal length.
Question
What is the unit of power of a lens?
Answer
Dioptre ($D$)
Question
State the formula for the power of a lens ($P$) when the focal length ($f$) is in metres.
Answer
$P = \frac{1}{f}$
Question
A lens has a power of $+2.0\ D$. What kind of lens is it?
Answer
Convex lens
Question
A lens has a power of $-2.0\ D$. What kind of lens is it?
Answer
Concave lens
Question
If two thin lenses of power $P_1$ and $P_2$ are placed in contact, what is the total power $P$ of the combination?
Answer
$P = P_1 + P_2$
Question
What is a 'simple microscope'?
Answer
A convex lens of short focal length mounted in a lens holder.
Question
What is the 'least distance of distinct vision' ($D$) for a normal human eye?
Answer
$25\ \text{cm}$
Question
In a simple microscope, where should the object be placed to see a clear magnified image?
Answer
Between the optical centre and the focus of the lens.
Question
What is the formula for the magnifying power ($M$) of a simple microscope?
Answer
$M = 1 + \frac{D}{f}$
Question
Which type of lens is used in spectacles to correct short-sightedness (myopia)?
Answer
Concave lens
Question
Which type of lens is used in spectacles to correct long-sightedness (hypermetropia)?
Answer
Convex lens
Question
What is the purpose of the objective lens in a telescope or camera?
Answer
To form a real and inverted image of the object.
Question
In the 'distant object method' for finding the focal length of a convex lens, where is the image formed?
Answer
In the focal plane of the lens.
Question
In the 'auxiliary plane mirror method', why is the distance of the object pin from the lens equal to its focal length when parallax is removed?
Answer
Because rays from the pin become parallel after refraction, strike the mirror normally, and return to form an image at the pin's own position.
Question
How can you differentiate between a convex and a concave lens by simply touching them?
Answer
A convex lens is thick in the middle and thin at the edges, while a concave lens is thin in the middle and thick at the edges.
Question
How can you differentiate between a convex and a concave lens by seeing the image of a printed page held near them?
Answer
If the letters appear magnified, it is a convex lens; if they appear diminished, it is a concave lens.
Question
What property of a lens is utilized when it is used as a 'burning glass'?
Answer
Its ability to converge parallel rays from the sun to a single point at its focus.
Question
In the lens formula, $v$ represents the distance of the _____ from the optical centre.
Answer
Image
Question
A lens with higher power has a _____ focal length than a lens with lower power.
Answer
Shorter
Question
The focal length of a convex lens is $20\ \text{cm}$. What is its power in dioptres?
Answer
$+5.0\ D$
Question
If an object is placed at the focus of a convex lens, where is the image formed?
Answer
At infinity
Question
What is the magnification of an image if it is the same size as the object?
Answer
$1$ (or $-1$ if inverted)
Question
Why is the second focal point usually referred to as 'the focus' of a lens?
Answer
Because it is the point where rays originally parallel to the principal axis converge (convex) or appear to diverge from (concave).
Question
For a thin lens, the radius of curvature of a surface is equal to the distance of the _____ from the optical centre.
Answer
Centre of curvature