Focal length Definition and 236 Threads

Homework Statement A biology student wishes to examine a bug at a magnificaiton of +5.00 by looking through a single lens. the student wants the image of the bug to be a distance of 25cm from the lens. what is the focal length of the lens? Answer:+6.25cm Homework Equations...

Homework Statement For my high school physics coursework I must investigate factors affecting the focal length of a lens. I have focused on radii of curvature and completed my data collection and verified the accuracy using the lens makers equation. However, in the conclusion I am really...

Homework Statement My physics coursework hypothesis: "As the thickness of the lens increases, the focal length decreases". I tested this using a ray box, a stand to hold lenses and a screen to see where the light is most focused, which is then measured with a metre ruler. The distance 'u'...

Homework Statement "An object 10 cm tall is placed at the zero mark of a meter stick. A spherical mirror located at some point on the meter stick creates an image of the object that is upright, 4 cm tall, and located at the 42 cm mark of the meter stick. Where is the mirror located and what...

Homework Statement The image obtained with a converging lens is upright and 3x the length of the object. The focal length of the lens is 20cm. Calculate the object and image distances Homework Equations I have absolutely no idea how to even start this question The Attempt at a...

Homework Statement In Sears' Optics, chapter 4, Sears claims that the first and second focal lengths (distance from first and second focal points to the first and second principal points, respectively) are equal, and he seems to imply that this is true in general, without proof. I am a...

Homework Statement Derive an expression for the Fresnel Zone Plate's focal length Homework Equations The fresnel zone plate has a transmittance function 1/2 (1+cos (ar^2)circ(r/2l)The Attempt at a Solution Using Eulers formula. this splits into e^(iar^2)+e^(-iar^2) Since the transmittance...

Any help much appreciated - this is driving me crazy! Homework Statement Show that the focal length of a half ball lens, in air, (in the paraxial limit) is given by: f=R/n-1 Homework Equations Equation (from Hecht- Optics), for refraction at a spherical interface: nm/s0 + nl/s1...

My question is as follows: Does the focal length of a concave lens change when put into a medium such as water? Initially the focal length of the lens is 20 cm in air. Would it get larger or small in water?

Hello! I am preparing an exam about optics and I can't understand a formula.my problem is at the end of the second page (the explanation is in Italian, but the formulas are easy to read for everybody). I can't understand how in the last step f=R/2 - (1+b) * r^2/4R etc they found -(1+b)...

Lenses - magnification, position, focal length help! ok so here is the quesiton.. An object is loacted 3.5 cm from the optical centre of a lens. The lense produces an image of magnification that is +0.667 the size of the object a) is this lens a diverging or converging lens? explain b)...

I am a bit of confused with the following terms involved in the study of lenses: Diameter of lens Aperture of Lens Are these terms synonomous? And how are they related to focal length of a lens?

is it true when we use concave lens, we can obtain focal length when we get the sharp image.. is it possible to get more than one sharp image when we change the object distance (distance between object and lens)

Hello all. I am in the process of designing a mechanism for collimating laser diodes for a personal project. I don't have training in optics so I am stuck with this problem: I need to determine the focal length of a lens that can be used to collimate a laser beam with divergence 45 degrees...

Homework Statement A telescope has a focal ratio of f/7.5. You wish to use it with a spectrometer that requires an f/10 beam at its input. Compute the focal length of a 50mm diameter lens that, when inserted in the beam 150mm in front of the unmodified focal plane, produces the required beam...

Hi I am trying to design a three mirror off axis telescope. The free parameters are the offset between the apertures and distance betwen the surfaces. My question is for a system with 'n' elements/surfaces (n=3 in my case) is there a relation between optical path length and focal length of...

Hi all, I'm looking for an equation which will give me the focal length of a biconvex lens given that we know both Radii of curvature, the thickness of the lens and the refractive index inside and outside. An equation is given on wikipedia here http://en.wikipedia.org/wiki/Focal_length as I...

Homework Statement When an object is at distance u1 and u2 from pole of concave mirror, images of same size are formed. Find the focal length of the mirror. Homework Equations 1/v+1/u=1/f The Attempt at a Solution Can u please ray diagram for this question. I think their is only...

From what I recall, determining the focal length of a double convex lens just involves an infinite light source and holding the lens close to an image-capturing surface such as a wall and adjusting the distance until the image is at its sharpest...the distance from the lens to the surface is the...

I am trying to find the focal length of a convex lens, so i let the sun shine through my window to create an image on my wall. Is that image the focal point or is it just the distance of the image? And if it is the distance of the image, how do I put it into the thin lens equation, since the...

So I bought a magnifying glass for a project and I need to find out the focal length of it in order to use it. I'm given the magnification of the converging lens, which is 5, but I don't any methods or how I could find out the focal length given only the magnification. I attempted to set...

A diverging lens is used to form a virtual image of an object. The object is 88.0 cm in front of the lens, and the image is 44.0 cm in front of the lens. Determine the focal length of the lens. 1/f = 1/do + 1/di since do is 88cm infront of the lense, that would keep the 88 positive, and...

Homework Statement Suppose that you wish to construct a telescope that can resolve features 7.0 km across on the Moon, 384,000 km away. You have a 2.2 m-focal-length objective lens whose diameter is 10.5 cm. What focal-length eyepiece is needed if your eye can resolve objects 0.10 mm apart...

Homework Statement For a lens with a focal length f, find the smallest distance possible between the object and its real image. Homework Equations 1/s + 1/s' = 1/f The Attempt at a Solution I tried plugging in different numbers for the solution, but I'm not sure how to reach the...

Homework Statement A person had a near point of 80 cm. What focal length lens would be required to correct this persons near point to 30cm, allowing then to read at this distance Homework Equations Maybe: 1/di + 1/do = 1/f = P I found another similar question which used 1/(the...

In my Optics book (Hecht, 4th edition) the author discusses how, when dealing with a multiple lens system (say, with n many lenses), one can think of the system as a single lens system, with an effective focal length given by: 1/f(effective) = 1/f1 + 1/f2 + ... + 1/fn My question is this...

Is it possible to build a large mirror array (around 10 km²) with adjustable focal length (from 10 meters to around 400 kilometers)?

Homework Statement A lens forms an image of an object. The object is 16 cm from the lens. The image is 12 cm from the lens on the same side as the object. (a) What is the focal length of the lens? Is the lens converging or diverging? (b) If the object is 8.5 mm tall, how tall is the image...

I know that for concave lenses, the focal length is one half the length of the radius of the circle. Is this also true for convex lenses? Thanks

I am trying to design a microscope and I am confused about some concepts. I plan to buy a Olympus 4x infinite-corrected objective lens and 6 inch long tube and a tube lens (f= 75mm) and then a CCD camera. By chance I found a tutorial online...

Hi guys! I face some difficulty in finding the focal length of a microlens. Assuming that the microlens is spherical, and the medium of the surrounding is vacuum. I really need an equation to get me going for my research task. I actually found the lensmaker's equation which is /=(−)(/−/+(−)/...

Homework Statement Following data recorded: Di (distance image to lens) 6.0 8.0 10.0 12.0 14.0 Magnification 0.2 0.6 1.0 1.4 1.8 Graph magnification against di and use the graph to find the focal length of the lens. Homework Equations...

Homework Statement A patient is unable to see objects closer than 1m from the eye. Calculate the focal length of the converging lens which is required to reduce this to 25cm. Homework Equations Lensmaker's equation: 1/u + 1/v = 1/f u= distance from object to lens v= distance of...

Homework Statement Three “thin” lenses, each with focal length 25 mm, are placed 5 cm from each other. What is system focal length? Homework Equations Lens equation: (1/oxd) + (1/ixd) = i/fx Oxd = object distance Ixd = image distance fx = focal The Attempt at a Solution...

What is the longest focal length practical for a lens and/or shaped mirror? I ask because it would be good if we could put a mirror at GEO and shine sunlight at one spot on the Earth 24 hours per day for energy production. The problem is sunlight is dispersing 1% so from 36,000km it would...

Homework Statement Find focal length for meniscus convex (3) Given r = 15, and n = 1.5 Homework Equations Lens maker's equation The Attempt at a Solution I can find the rest except #3 and #6 (they are opposite sign of each other) I had (1/r1 - 1/r2) gives zero... which is wrong... I...

A magnifying glass has a diverging lens with a 13.0 cm focal length. At what distance from a toothpick should you hold this lens to form an image with a magnification of +5.00? I simply have no idea what to do. I tied moving around the equations to make them work, but I can't figure it out...

Homework Statement A 2.0 cm tall object is placed in front of a mirror. A 1.0 cm tall upright image is formed behind the mirror, 165 from the object Homework Equations 1/s + 1/s' = 1/focal length h' / h = magnification -s' / s = magnification h' / h = s' / s (i found this...

Homework Statement Derive 1/f = (n-1)*( (1/r1)-(1/r2)+(((n-1)t))/(nr1r2))) using the ray matrix method. Homework Equations Matrix for curved dielectric interface: M1 = [1, 0] [(1/r)(n1/n2 - 1), n1/n2 ] Matrix for translation through homogeneous medium: M2 = [1, d] [0...

Homework Statement A thin lens has radii of curvature R1=-8.00 cm and R2=-6.00 cm, indices of refraction n(red)=1.50 and n(violet)=1.53 for red and violet light, and is surrounded by air. An object at distance s=20.0 cm from the lens has an image at distance s' from the lens. Is there a second...

Homework Statement I must calculate the focal length of a GRIN lens. I.e. a lens which radii of curvature are both infinite. I will consider the refractive index to be varying in function of y (the height in the x-y plane). Say I'm given that at y=0, n(0)=n_0. I'm not sure how to write...

Homework Statement http://img256.imageshack.us/img256/6330/35727193.jpg Homework Equations [PLAIN]http://upload.wikimedia.org/math/2/0/a/20aa1e3d192ecb3164ac4f2095c86cd3.png The Attempt at a Solution i have used triangle from up side of h1 to the b. and by using sinß and...

Homework Statement What is the focal length of a drop of water in the form of a sphere with radius 4mm given that the refractive index of water = 4/3 Homework Equations The Attempt at a Solution Here, i think i will need to assume that the light rays are coming from infinity...

1)Why when a convex lens having radius of curvature R, focal length f and refractive index μ is bisected horizontally along the principal axis its focal length remains the same, whereas when it is bisected vertically focal length becomes 2f? Does the same thing happens in the case of...

1. Why is a telephoto lens considered to have a longer focal length than that of a single convex lens? A lens combination, such as a diverging and a convex lens, is referred to as a telephoto lens. Suggest why a telephoto lens is considered to have a longer focal length than that of a single...

1. Why is a telephoto lens considered to have a longer focal length than that of a single convex lens? A lens combination, such as a diverging and a convex lens, is referred to as a telephoto lens. Suggest why a telephoto lens is considered to have a longer focal length than that of a single...

Homework Statement An object is placed in front of a concave mirror, 15.0 cm from the mirror's focal point. The image formed by the mirror is five times farther away from the focal point. Calculate the focal length of the mirror. There are actually two possible image distances di that...

Homework Statement A slide projector needs to create a 81 cm high image of a 2.0 cm tall slide. The screen is 360 cm from the slide. Assume that it is a thin lens. What focal length does the lens need? How far should you place the lens from the slide? Homework Equations 1/f = 1/s +...

Homework Statement A Lens maker desires to make a lens of focal length 30 cm. in air. The equipment in the shop is set to make lens faces of 20 cm. a. What index of refraction should the lens material have to produce this focal length? b. What would the focal length of this lens be...

Homework Statement Hey,, I was wondering if anyone could help me with some physics work i am doing as i am completely stuck. Is there supposed to be any relationship with the magnification and focal length of lenses and if so what is it? I did an experiment and measure the magnification of...