What is the **lens formula** used for refraction through lenses?
Analysis & Theory
The correct **lens formula** is: **1/f = 1/v - 1/u**, where f = focal length, v = image distance, and u = object distance.
What is the sign of focal length for a **convex lens**?
Analysis & Theory
A **convex lens** is a **converging lens**, so its **focal length is positive**.
What is the nature of the image formed by a **concave lens**?
A
Real, inverted, and magnified
B
Virtual, erect, and diminished
C
Real, erect, and enlarged
D
Virtual, inverted, and enlarged
Analysis & Theory
**Concave lenses** always form **virtual, erect, and diminished** images.
What happens when an object is placed **at 2F** in front of a **convex lens**?
B
Image at 2F, same size and real
C
Image beyond 2F, diminished
D
Image behind the object, virtual
Analysis & Theory
Object at **2F** in a convex lens forms an image at **2F**, which is **real, inverted, and same size**.
Which lens is used in **magnifying glasses**?
Analysis & Theory
**Convex lenses** are used as **magnifying glasses** because they can form **enlarged virtual images** when object is within focal length.
If focal length `f = 10 cm` and object distance `u = -15 cm`, find the image distance `v` using the lens formula.
Analysis & Theory
Using 1/f = 1/v - 1/u → 1/10 = 1/v - (-1/15) → 1/v = 1/10 - (-1/15) = (3 + 2)/30 = 5/30 → v = 6 cm. (Positive = real and on opposite side).
Which of the following is true for a **real image** formed by a lens?
A
Cannot be captured on screen
C
Always on the same side as object
D
Can be projected on a screen
Analysis & Theory
**Real images** can be **projected on a screen**, as light rays actually meet at the image location.
What does a **negative image distance (v)** signify in a lens?
B
Virtual image on same side as object
Analysis & Theory
A **negative image distance** in lens formula means the image is **virtual and on the same side** as the object.
A ray of light parallel to the principal axis after refraction through a **convex lens** passes through:
C
Focus on the opposite side
Analysis & Theory
In a **convex lens**, a ray **parallel to the principal axis** refracts through the **focus on the opposite side**.
Which lens would you use to **correct short-sightedness (myopia)**?
Analysis & Theory
**Concave lenses** are used to correct **myopia** as they diverge light rays and form the image on the retina.