If the retinal image decreases in size, what does this indicate about the object's motion?

If the retinal image decreases in size, what does this indicate about the object's motion?

• A. The object is moving closer
• B. The object is moving away
• C. The object's distance is constant
• D. The object's size is increasing

Answer: (B)

When the image on the retina becomes smaller, the angular size of the object is shrinking. That happens as distance increases: the same object subtends a smaller angle at the eye, so its retinal image decreases in size. If the distance stays constant, the retinal image would stay the same size; if the object were moving closer, the retinal image would grow rather than shrink. If the object's actual size increased, the retinal image would also tend to grow. So the decreasing retinal size indicates the object is moving away.

When the image on the retina becomes smaller, the angular size of the object is shrinking. That happens as distance increases: the same object subtends a smaller angle at the eye, so its retinal image decreases in size. If the distance stays constant, the retinal image would stay the same size; if the object were moving closer, the retinal image would grow rather than shrink. If the object's actual size increased, the retinal image would also tend to grow. So the decreasing retinal size indicates the object is moving away.