# KhanAcademyVideos

### Beyond the Madonna, an early image of enslaved people in Renaissance Florence

Filippino Lippi, Madonna and Child, c. 1483–84, tempera, oil, and gold on wood, 81.3 x 59.7 cm (The Metropolitan Museum of Art), an Expanding Renaissance Initiative video, find more at https://Smarthistory.org
Speakers: Dr. Lauren Kilroy-Ewbank and Dr. Beth Harris

### Snell's laws proof using Huygen's principle

Huygen's principle states that every point on a wavefront behaves as a source for secondary waves, whose common tangent (envelop) becomes the new wavefront. Using this principle, let's prove Snell's law of refraction - Ratio of the sine of angle of incidence to the sine of the angle of refraction is a constant.

### Angle between wavefront & rays of light

Wavefronts are always perpendicular to the rays of light, always! Let's explore why.

### Intensity in YDSE (Visual method-phasors) I =4Io cos^2(phi/2)

Let's calculate the expression for the intensity of interfering waves due to coherent sources. The expression turns out to be I =4 Io cos^2 (phi/2)

### Huygen's principle of secondary waves

Huygen's principle states every point on the current wavefront acts as a source of secondary spherical waves. These secondary waves propagate outwards, in the forward direction, and a common tangent (an envelope) to all these waves constitute the new wavefront.

### Huygen's theory of light & wavefronts

Huygen's proposed that light is a longitudinal wave in the aether medium - a medium that pervades the entire universe. The set of particles oscillating in phase with each other is called a wavefront. Point sources give out spherical wavefronts, while sources at infinity give plane wavefronts.

### A new art for a new nation: Félix Parra’s Bartolomé de las Casas

A conversation between Dr. Lauren Kilroy-Ewbank and Dr. Beth Harris in front of Félix Parra, Fray Bartolomé de las Casas, 1875, oil on canvas, 263 x 356.5 cm (Museo Nacional de Arte, Mexico City)

### Limits of composite functions: internal limit doesn't exist

Finding the limit of g(h(x)) at x=-1 when the limit of h(x) at x=-1 doesn't exist. Does it mean that the composite limit doesn't exist? Not necessarily! See how we analyze it.

### Theorem for limits of composite functions: when conditions aren't met

Suppose we are looking for the limit of the composite function f(g(x)) at x=a. This limit would be equal to the value of f(L), where L is the limit of g(x) at x=a, under two conditions. First, that the limit of g(x) at x=a exists (and if so, let's say it equals L). Second, that f is continuous at x=L. If one of these conditions isn't met, we can't assume the limit is f(L).

### Limits of composite functions: external limit doesn't exist

Finding the limit of g(h(x)) at x=1 when the limit of h(x) at x=1 is 2 and the limit of g(x) at x=2 doesn't exist. Does it mean that the composite limit doesn't exist? Not necessarily! See how we analyze it.

### Example: Graphing y=-cos(π⋅x)+1.5

Sal graphs y=-cos(π⋅x)+1.5 by thinking about the graph of y=cos(x) and analyzing how the graph (including the midline, amplitude, and period) changes as we perform function stransformations to get from y=cos(x) to y=-cos(π⋅x)+1.5.

### Example: Graphing y=3⋅sin(½⋅x)-2

Sal graphs y=3⋅sin(½⋅x)-2 by thinking about the graph of y=sin(x) and analyzing how the graph (including the midline, amplitude, and period) changes as we perform function stransformations to get from y=sin(x) to y=3⋅sin(½⋅x)-2.

### Sacred geometry in a Renaissance ceiling from Spain

A conversation with Dr. Lauren Kilroy-Ewbank and Dr. Steven Zucker below a mudéjar-style ceiling, 16th century, carved, painted, and gilded wood, 28 x 33 ft., Spain (The Metropolitan Museum of Art)

### A chalice from the Attarouthi Treasure

Evan Freeman and Anne McClanan, PhDs in Byzantine Art History, here discuss a Byzantine chalice, now at the Metropolitan Museum of Art (Acc. 1986.3.2).
Video Editor: Anna Weltner
This video is available CC BY 4.0
Here's info on the object from the Met's website:
https://www.metmuseum.org/art/collection/search/466136
Date: 500–650
Geography: Made in Attarouthi, Syria
Culture: Byzantine
Medium: Silver and gilded silver
Dimensions: Overall: 9 11/16 × 6 9/16 in., 16.7oz. (24.6 × 16.7 cm, 474g)
Diam. of foot: 3 15/16 in. (10 cm)
Diam. of knop: 1 5/8 in. (4.1 cm)
Capacity of cup: 2000 ml
With a youthful Christ with a cruciform halo, a deacon saint with censer (probably Saint Stephen), a youthful saint with staff, the Virgin
Mary in orant pose, a military saint in armor killing a dragon (Saint George ?), and a long-haired Saint John the Forerunner, under arcades
Inscribed in Greek: Of Saint Stephen of the village of Attaroutha
An unusual aspect of these chalices is their repeated representation of military saints. The figures in armor killing a dragon may be the earliest surviving depictions of Saint George, who according to tradition was martyred in the eastern Mediterranean in the fourth century or earlier.

### A Renaissance dinner service for a duchess

A conversation between Dr. Lauren Kilroy-Ewbank and Dr. Beth Harris in front of Nicola da Urbino, armorial plate (tondino), The Story of King Midas, c. 1520–25, tin-glazed earthenware, 27.5 cm in diameter (The Metropolitan Museum of Art)

### The cost of war: Delacroix, Greece on the Ruins of Missolonghi

Eugène Delacroix, Greece on the Ruins of Missolonghi, 1826, oil on canvas, 208 cm × 147 cm (Musée des Beaux-Arts de Bordeaux). Speakers: Dr. Steven Zucker and Dr. Beth Harris

### LCR resonance & resonant frequency

At the resonant frequency, the L.C.R. circuit has a minimum impedance and maximum current. Impedance is a minimum when capacitive reactance equals inductive reactance.

### Transformers - working & applications (step up and step down)

Transformers step up (increase) or step down (decrease) AC voltage using the principle of electromagnetic induction - mutual induction. A changing current in the primary coil induces an e.m.f in the secondary. Since the e.m.f generated depends on the number of turns, the voltage induced in the secondary can be changed - stepped up or down - by altering the turn's ratio.

### Huygen's principle - reflection laws proof

Huygen's principle states that every point on a wavefront behaves as a source for secondary waves, whose common tangent (envelop) becomes the new wavefront. Using this principle, let's prove the laws of reflection

### LCR frequency response & quality

The frequency response graph is a graph of current vs frequency, keeping all other variables a constant. For LCR circuits, the graph has a peak at the resonant frequency. This is used to tune radios and other devices to specific radio frequency channels. A high-quality LCR resonance has a narrow graph, while a low-quality has a flatter graph - the sharpness determines the quality.

## Páginas

## Tópicos ativos do fórum

## KhanAcademyExercises

## Apologética para el mundo - José Miguel Arráiz

## Mera defensa de la fe - Eleuterio Fernández Guzmán

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