The Thin Lens Equation
Section 11.3.b
Thin Lens Equation
The distance of the object from the lens do, the distance of the image form the lens di, and the focal length f, can all be related using the thin lens equation:
Keep in mind the following points when working with the thin lens equation:
A focal point on the same side as the object is a negative focal length (concave).
The image distance di is positive if the image is real (opposite side of object - only concave) and negative if the image is virtual (same side as object - concave or convex).
Example 1:
A convex lens of a magnifying glass is held 2.00 cm above a page to magnify the print. If the image produced by the lens is 3.60 cm away and virtual, what is the focal length of the magnifying glass?
Given
Distance of object from lens, do = 2.00 cm
Distance of virtual image from lens, di = -3.60 cm
Required
Focal length of lens, f = ?
Analysis & Solution
Paraphrase
The focal length is about 4.50 cm.
Example 2:
A convex lens has a focal length of 60.0 cm. A candle is placed 50 cm from the lens. What type of image is formed and how far is the image from the lens?
Given
Focal length of concave lens, f = 60 cm
Distance of object from lens, do = 50 cm
Required
Distance of image from lens, di = ?
Analysis & Solution
Paraphrase
Since di is negative, the image is virtual and is located 300 cm from the lens.
Example 3:
A camera with a 200 mm lens makes a real image of a bird on film. The film is located 201 mm behind the lens. Determine the distance from the lens to the bird.
Given
Focal length of lens, f = 200 mm
Distance of image/film from lens, di = 201 mm
Required
Distance of object/bird from lens, do = ?
Analysis & Solution
Paraphrase
The bird is 40.2 m away from the camera lens.
mm
Seatwork / Homework
Read Section 11.3 (p. 454 – 457)
Answer all 9 Practice Problems (p. 455 – 457)
Check and Reflect # 1 – 3, 6 – 8, 10, 11 (p. 462)
Chapter 11 Review (p. 464 & 465)