Welcome to You Ask Andy

Dewan Higgins, age 10, of Grand Island, Nebraska, for his question:


Why does a spoon reflect upside down?


No matter how you look at a shiny spoon it is bound to show a very unflattering reflection of your face. If you look at the bulging back of a spoon, you see a miniature face, right side up but with bulging nose and sunken cheeks. If you gaze inside the curved bowl you see a skinny upside down face, with the chin where the forehead should be.

Mirrored reflections are full of tricky angles. A smooth flat mirror shows a more or less true picture, though the sides are reversed. Your right ear is where your left ear should be. A curved mirror has even more tricks and if it happens to curve inward, it stands you on your head. The inside of a shiny spoon is a curved mirror of this sort  ¬which is why it shows you an upside down reflection.

Reflections are caused by invisible rays of light, which travel in straight lines. Normally they carry pictures of the scenery to our eyes. But when they strike a mirrored surface they. bounce back and turn around. This is why you can see your reflection in an ordinary flat mirror. Rays of light carry an image of your face to the shiny glass surface    and bounce straight back with a reflection.

All sharp turns are angles, or corners. And angles are the secrets behind all reflections. Basically there are two kinds. The angle of incidence is the angle at which light rays strike an object while the angle of reflection is the turn they take when they bounce back. And the angle of incidence always equals the angle of reflection. This is the secret that twists reflections out of shape.

The inside of a spoon is a concave mirror, scooped out like a smooth, shiny little cave. Naturally the rays of light run into some very tricky problems when they strike those curved concave sides. They cannot simply strike the surface and reflect straight back, as they do from an ordinary flat mirror.


Actually, the light rays are forced to strike the sloping sides at sloping angles. And remember, these angles of incidence must equal the angles of reflection. This means that the reflected rays are turned around at sloping angles. Those that came from your chin are angled upward. Those that came from your forehead are angled downward.

All this magical turn about is invisible. The reflected rays from the spoon are sloped and angled towards each other. They meet at a certain point above the shiny surface, but still inside the bowl of the spoon. At this invisible point the lines cross over    and keep on going. This is where the reflections from the top change places with those at the bottom. When the picture reaches your eyes, the reflection is upside down.

Reflections are hard to understand because we cannot see rays of light. We see only the pictures they bring to our eyes. When they strike a shiny surface, they bounce back at invisible angles. And the concave bowl of a spoon makes the reflected rays slope and cross over. Those from the forehead switch places with those from the chin    and the reflected face is upside down.

 

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