Lenses- Physics

Question

An object is placed 45 cm in front of a converging lens with a focal length of 20 cm. Draw a ray diagram. Estimate the image distance and give the image characteristics.

the answer the book gives is: D=36 cm, real, inverted, reduced.

I am not too sure how to get that answer.(please use units)

Answer 1

Hi Marybel! So to get the image distance, we need to use the thin lens equation. This equation states that:

1/o + 1/i = 1/f, where o = the object distance, i = the image distance, and f = the focal length of the lens.

Using this formula, we can plug in our known values to get:

1/45 + 1/i = 1/20 (all values in cm)

And solve for i to get i = 36 cm.

The other parts of the answer deal with properties of using a converging lens and having an object that is outside the focal point (positioned greater than one focal point length away from the lens). In this case, the image of the object will be real, inverted, and reduced. Drawing a ray diagram of the situation can really help you see why this is true. I hope this helped!