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examBoard: AQA
examType: GCSE
lessonTitle: Relative Size
Imagine you're looking at a photo of two identical cars on a road. One appears much smaller than the other. Without thinking, you immediately know which car is further away. This automatic judgement happens because your brain uses relative size to determine distance. It's one of the most important visual cues we use every day!
Key Definitions:
When two objects are known to be the same size (like two identical cars), the one that creates a smaller image on your retina is automatically perceived as being further away. This happens because objects appear smaller as they get further from us - the image they cast on our retina shrinks with distance.
We use relative size constantly! When you see a row of lamp posts along a road getting progressively smaller, you don't think they're actually shrinking in size - you understand they're getting further away. Similarly, when you see a tiny aeroplane in the sky, you don't think it's actually small - you know it's far away.
Relative size works because of how light travels to our eyes. Objects at different distances create different-sized images on our retina, even when the objects themselves are identical in size. Our visual system has evolved to interpret these size differences as information about distance.
When light from an object enters your eye, it forms an image on your retina. The size of this retinal image depends on two factors:
Larger objects naturally create larger retinal images than smaller objects at the same distance.
The further away an object is, the smaller its retinal image becomes, following the principles of optical geometry.
Your brain combines information about retinal image size with your knowledge about objects to judge distance.
Relative size works hand-in-hand with another important perceptual phenomenon called size constancy. Size constancy is our ability to perceive objects as maintaining the same size despite changes in viewing distance.
Size constancy means that even though the retinal image of your friend gets smaller as they walk away from you, you don't perceive them as shrinking. Your brain automatically compensates for the distance, maintaining a constant perception of their actual size.
Relative size and size constancy work together. Your brain uses relative size to judge distances and then applies size constancy to maintain accurate perceptions of object sizes at those distances. Without size constancy, relative size cues would lead to a very confusing visual world!
Understanding relative size has important applications in many fields:
Artists and photographers manipulate relative size to create the illusion of depth in two-dimensional images. By making objects in the "distance" smaller, they create realistic-looking scenes on flat canvases or photographs.
Renaissance painters used relative size to create realistic perspective in their paintings, making distant objects proportionally smaller.
Photographers use relative size effects to create forced perspective illusions, like tourists "holding up" the Leaning Tower of Pisa.
Filmmakers use relative size in special effects to make models appear full-sized or to create the illusion of enormous creatures.
The Ames Room is a specially constructed room that manipulates relative size cues to create a powerful illusion. The room is actually trapezoid-shaped, but appears rectangular when viewed through a peephole. When two people stand in different corners, one appears giant while the other looks tiny! This works because our brain assumes the room is rectangular and interprets the different sizes of the people as being due to different distances, when they're actually standing at different distances from the viewer. This demonstrates how strongly our visual system relies on relative size for distance perception.
Psychologists have conducted various studies to understand how relative size influences our perception:
Psychologist James Gibson conducted pioneering research on depth perception in the 1950s. He identified relative size as one of the most important monocular depth cues, showing that it works even when other depth cues are absent. Gibson demonstrated that people could accurately judge distances based solely on relative size information.
Research has shown that relative size perception works similarly across different cultures, suggesting it's a fundamental aspect of human visual processing rather than something learned through cultural experience. Even people from cultures with very different visual environments use relative size to judge distance.
While relative size is a powerful depth cue, it does have some limitations:
Have you ever noticed that the moon looks much larger when it's near the horizon compared to when it's high in the sky? This is called the Moon Illusion and it's partly related to relative size perception. When the moon is near the horizon, we see it alongside distant landmarks and our brain interprets it as being further away. Due to size constancy, we perceive it as larger. When it's high in the sky without reference objects, this effect doesn't occur. In reality, the moon's retinal image size is exactly the same in both positions!
Relative size is a fundamental aspect of how we perceive the world around us. It allows us to:
By understanding how relative size works, you can better appreciate how your visual system creates your perception of the world and how easily this perception can sometimes be tricked by illusions that manipulate these basic visual cues.
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