What is the focal length of a lens if it forms an image at infinity when the object is placed at 30 cm?
Practice Questions
1 question
Q1
What is the focal length of a lens if it forms an image at infinity when the object is placed at 30 cm?
30 cm
15 cm
60 cm
20 cm
For the image to be at infinity, the focal length must be equal to the object distance, hence f = 60 cm.
Questions & Step-by-step Solutions
1 item
Q
Q: What is the focal length of a lens if it forms an image at infinity when the object is placed at 30 cm?
Solution: For the image to be at infinity, the focal length must be equal to the object distance, hence f = 60 cm.
Steps: 5
Step 1: Understand that when an object is placed at a certain distance from a lens, it can form an image at a specific location.
Step 2: Recognize that if the image is formed at infinity, it means the light rays coming from the object are parallel when they reach the lens.
Step 3: Know that for a lens to form an image at infinity, the focal length (f) of the lens must be equal to the object distance (d) when the object is placed at a certain distance.
Step 4: In this case, the object is placed at 30 cm, so we set the focal length equal to this distance.
Step 5: Therefore, the focal length of the lens is f = 30 cm.
