Where is the "front" of a lens?

[Buttermellow]

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All the positive/negative conventions consider which side of the lens is the "front." Is it always the side the observer is on? For example, in question 953 of EK 1001 Physics:

A lens makes a 3cm inverted image of a 6cm object when the object is placed 10cm from the lens. Where is the image formed?

A. 5 cm in front of the lens
B. 5 cm behind the lens
C. 20 cm in front of the lens
D. 20 cm behind the lens

Their explanation of the answer is " B is correct. h(i) = -3, h(o) = 6, d(o) = 10. The distance is positive, so the image is behind the lens (on the observer side)."

I guess my question is, how did they know that h(i) is negative? If somebody could explain these conventions to me, that would be awesome!

 

[Jepstein30]

All the positive/negative conventions consider which side of the lens is the "front." Is it always the side the observer is on? For example, in question 953 of EK 1001 Physics:

A lens makes a 3cm inverted image of a 6cm object when the object is placed 10cm from the lens. Where is the image formed?

A. 5 cm in front of the lens
B. 5 cm behind the lens
C. 20 cm in front of the lens
D. 20 cm behind the lens

Their explanation of the answer is " B is correct. h(i) = -3, h(o) = 6, d(o) = 10. The distance is positive, so the image is behind the lens (on the observer side)."

I guess my question is, how did they know that h(i) is negative? If somebody could explain these conventions to me, that would be awesome!

see below, i had some errors 😉

 

[V5RED]

front = opposite observer (as EK says, you stand BEHIND a camera lens)

Smaller, inverted image points to a diverging (concave) lens so object will be virtual, on the observer's side and same side as the object. Real would be on the other side.

Also, hi is negative because its inverted, as it says in the stem.

m = -hi/ho = di/do = 3/6 = 1/2 so di = 5 cm

You have some stuff backwards.

Inverted=Real=converging=convex lens(within the focal distance a convex lens switches to virtual though)

Upright=Virtual=concave lens

 

[Jepstein30]

You have some stuff backwards.

Inverted=Real=converging=convex lens(within the focal distance a convex lens switches to virtual though)

Upright=Virtual=concave lens

converging lens -> convex lens -> concave mirror -> SIR (small, inverted, real) to LUV (larger, upright, virtual)

diverging lens -> concave lens -> convex mirror -> SUV (small, upright, virtual)

yea my bad.. wrote that without thinking through

 

[Buttermellow]

converging lens -> convex lens -> concave mirror -> SIR (small, inverted, real) to LUV (larger, upright, virtual)

diverging lens -> concave lens -> convex mirror -> SUV (small, upright, virtual)

yea my bad.. wrote that without thinking through

I've got that part down pretty well, I'm just really confused as to how examkrackers got a negative image height. If you know the image will be inverted, do you make the height negative in the thin lens equation?

 

[V5RED]

I've got that part down pretty well, I'm just really confused as to how examkrackers got a negative image height. If you know the image will be inverted, do you make the height negative in the thin lens equation?

Inverted=negative magnification.

M=hi/ho
ho is always positive
hi is negative if the image is inverted, positive if it is upright

You make it negative in the equation by adding a minus sign.

 

[Buttermellow]

Inverted=negative magnification.

M=hi/ho
ho is always positive
hi is negative if the image is inverted, positive if it is upright

You make it negative in the equation by adding a minus sign.

Thanks! That's exactly what I needed clarified.

 

[sciencebooks]

Thanks! That's exactly what I needed clarified.

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